diff --git a/README.md b/README.md
index 2ebbf8d..bed4154 100644
--- a/README.md
+++ b/README.md
@@ -166,6 +166,19 @@ fastify.register(require('fastify-basic-auth'), {
 })
 ```
 
+The `realm` key could also be a function:
+
+```js
+fastify.register(require('fastify-basic-auth'), {
+  validate,
+  authenticate: {
+    realm(req) {
+      return 'example' // WWW-Authenticate: Basic realm="example"
+    }
+  }
+})
+```
+
 
 ## License
 
diff --git a/index.js b/index.js
index 561f33a..0b637a7 100644
--- a/index.js
+++ b/index.js
@@ -4,20 +4,15 @@ const fp = require('fastify-plugin')
 const auth = require('basic-auth')
 const { Unauthorized } = require('http-errors')
 
-function basicPlugin (fastify, opts, next) {
+async function basicPlugin (fastify, opts) {
   if (typeof opts.validate !== 'function') {
-    return next(new Error('Basic Auth: Missing validate function'))
+    throw new Error('Basic Auth: Missing validate function')
   }
-  const authenticateHeader = getAuthenticateHeader(opts.authenticate, next)
+  const authenticateHeader = getAuthenticateHeader(opts.authenticate)
   const validate = opts.validate.bind(fastify)
   fastify.decorate('basicAuth', basicAuth)
 
-  next()
-
   function basicAuth (req, reply, next) {
-    if (authenticateHeader) {
-      reply.header(authenticateHeader.key, authenticateHeader.value)
-    }
     const credentials = auth(req)
     if (credentials == null) {
       done(new Unauthorized('Missing or bad formatted authorization header'))
@@ -34,6 +29,17 @@ function basicPlugin (fastify, opts, next) {
         if (!err.statusCode) {
           err.statusCode = 401
         }
+
+        if (err.statusCode === 401) {
+          switch (typeof authenticateHeader) {
+            case 'string':
+              reply.header('WWW-Authenticate', authenticateHeader)
+              break
+            case 'function':
+              reply.header('WWW-Authenticate', authenticateHeader(req))
+              break
+          }
+        }
         next(err)
       } else {
         next()
@@ -42,25 +48,28 @@ function basicPlugin (fastify, opts, next) {
   }
 }
 
-function getAuthenticateHeader (authenticate, next) {
+function getAuthenticateHeader (authenticate) {
   if (!authenticate) return false
   if (authenticate === true) {
-    return {
-      key: 'WWW-Authenticate',
-      value: 'Basic'
-    }
+    return 'Basic'
   }
   if (typeof authenticate === 'object') {
-    const realm = (authenticate.realm && typeof authenticate.realm === 'string')
-      ? authenticate.realm
-      : ''
-    return {
-      key: 'WWW-Authenticate',
-      value: 'Basic' + (realm ? ` realm="${realm}"` : '')
+    const realm = authenticate.realm
+    switch (typeof realm) {
+      case 'undefined':
+        return 'Basic'
+      case 'boolean':
+        return 'Basic'
+      case 'string':
+        return `Basic realm="${realm}"`
+      case 'function':
+        return function (req) {
+          return `Basic realm="${realm(req)}"`
+        }
     }
   }
 
-  next(new Error('Basic Auth: Invalid authenticate option'))
+  throw new Error('Basic Auth: Invalid authenticate option')
 }
 
 module.exports = fp(basicPlugin, {
diff --git a/test.js b/test.js
index c2b9d0f..33ce6c3 100644
--- a/test.js
+++ b/test.js
@@ -165,7 +165,7 @@ test('Basic with promises - 401', t => {
 })
 
 test('WWW-Authenticate (authenticate: true)', t => {
-  t.plan(3)
+  t.plan(6)
 
   const fastify = Fastify()
   const authenticate = true
@@ -190,6 +190,15 @@ test('WWW-Authenticate (authenticate: true)', t => {
     })
   })
 
+  fastify.inject({
+    url: '/',
+    method: 'GET'
+  }, (err, res) => {
+    t.error(err)
+    t.equal(res.headers['www-authenticate'], 'Basic')
+    t.equal(res.statusCode, 401)
+  })
+
   fastify.inject({
     url: '/',
     method: 'GET',
@@ -197,14 +206,14 @@ test('WWW-Authenticate (authenticate: true)', t => {
       authorization: basicAuthHeader('user', 'pwd')
     }
   }, (err, res) => {
-    t.equal(res.headers['www-authenticate'], 'Basic')
     t.error(err)
+    t.equal(res.headers['www-authenticate'], undefined)
     t.equal(res.statusCode, 200)
   })
 })
 
 test('WWW-Authenticate Realm (authenticate: {realm: "example"})', t => {
-  t.plan(3)
+  t.plan(6)
 
   const fastify = Fastify()
   const authenticate = { realm: 'example' }
@@ -229,6 +238,15 @@ test('WWW-Authenticate Realm (authenticate: {realm: "example"})', t => {
     })
   })
 
+  fastify.inject({
+    url: '/',
+    method: 'GET'
+  }, (err, res) => {
+    t.error(err)
+    t.equal(res.headers['www-authenticate'], 'Basic realm="example"')
+    t.equal(res.statusCode, 401)
+  })
+
   fastify.inject({
     url: '/',
     method: 'GET',
@@ -236,8 +254,8 @@ test('WWW-Authenticate Realm (authenticate: {realm: "example"})', t => {
       authorization: basicAuthHeader('user', 'pwd')
     }
   }, (err, res) => {
-    t.equal(res.headers['www-authenticate'], 'Basic realm="example"')
     t.error(err)
+    t.equal(res.headers['www-authenticate'], undefined)
     t.equal(res.statusCode, 200)
   })
 })
@@ -572,7 +590,7 @@ test('Invalid options (authenticate)', t => {
 })
 
 test('Invalid options (authenticate realm)', t => {
-  t.plan(3)
+  t.plan(6)
 
   const fastify = Fastify()
   fastify
@@ -597,6 +615,15 @@ test('Invalid options (authenticate realm)', t => {
     })
   })
 
+  fastify.inject({
+    url: '/',
+    method: 'GET'
+  }, (err, res) => {
+    t.error(err)
+    t.equal(res.headers['www-authenticate'], 'Basic')
+    t.equal(res.statusCode, 401)
+  })
+
   fastify.inject({
     url: '/',
     method: 'GET',
@@ -604,12 +631,154 @@ test('Invalid options (authenticate realm)', t => {
       authorization: basicAuthHeader('user', 'pwd')
     }
   }, (err, res) => {
+    t.error(err)
+    t.equal(res.headers['www-authenticate'], undefined)
+    t.equal(res.statusCode, 200)
+  })
+})
+
+test('Invalid options (authenticate realm = undefined)', t => {
+  t.plan(6)
+
+  const fastify = Fastify()
+  fastify
+    .register(basicAuth, { validate, authenticate: { realm: undefined } })
+
+  function validate (username, password, req, res, done) {
+    if (username === 'user' && password === 'pwd') {
+      done()
+    } else {
+      done(new Error('Unauthorized'))
+    }
+  }
+
+  fastify.after(() => {
+    fastify.route({
+      method: 'GET',
+      url: '/',
+      preHandler: fastify.basicAuth,
+      handler: (req, reply) => {
+        reply.send({ hello: 'world' })
+      }
+    })
+  })
+
+  fastify.inject({
+    url: '/',
+    method: 'GET'
+  }, (err, res) => {
+    t.error(err)
     t.equal(res.headers['www-authenticate'], 'Basic')
+    t.equal(res.statusCode, 401)
+  })
+
+  fastify.inject({
+    url: '/',
+    method: 'GET',
+    headers: {
+      authorization: basicAuthHeader('user', 'pwd')
+    }
+  }, (err, res) => {
+    t.error(err)
+    t.equal(res.headers['www-authenticate'], undefined)
+    t.equal(res.statusCode, 200)
+  })
+})
+
+test('WWW-Authenticate Realm (authenticate: {realm (req) { }})', t => {
+  t.plan(7)
+
+  const fastify = Fastify()
+  const authenticate = {
+    realm (req) {
+      t.equal(req.url, '/')
+      return 'root'
+    }
+  }
+  fastify.register(basicAuth, { validate, authenticate })
+
+  function validate (username, password, req, res, done) {
+    if (username === 'user' && password === 'pwd') {
+      done()
+    } else {
+      done(new Error('Unauthorized'))
+    }
+  }
+
+  fastify.after(() => {
+    fastify.route({
+      method: 'GET',
+      url: '/',
+      preHandler: fastify.basicAuth,
+      handler: (req, reply) => {
+        reply.send({ hello: 'world' })
+      }
+    })
+  })
+
+  fastify.inject({
+    url: '/',
+    method: 'GET'
+  }, (err, res) => {
+    t.error(err)
+    t.equal(res.headers['www-authenticate'], 'Basic realm="root"')
+    t.equal(res.statusCode, 401)
+  })
+
+  fastify.inject({
+    url: '/',
+    method: 'GET',
+    headers: {
+      authorization: basicAuthHeader('user', 'pwd')
+    }
+  }, (err, res) => {
     t.error(err)
+    t.equal(res.headers['www-authenticate'], undefined)
     t.equal(res.statusCode, 200)
   })
 })
 
+test('No 401 no realm', t => {
+  t.plan(4)
+
+  const fastify = Fastify()
+  fastify.register(basicAuth, { validate, authenticate: true })
+
+  function validate (username, password, req, res) {
+    const err = new Error('Winter is coming')
+    err.statusCode = 402
+    return Promise.reject(err)
+  }
+
+  fastify.after(() => {
+    fastify.route({
+      method: 'GET',
+      url: '/',
+      preHandler: fastify.basicAuth,
+      handler: (req, reply) => {
+        reply.send({ hello: 'world' })
+      }
+    })
+  })
+
+  fastify.inject({
+    url: '/',
+    method: 'GET',
+    headers: {
+      authorization: basicAuthHeader('user', 'pwdd')
+    }
+  }, (err, res) => {
+    t.error(err)
+    t.equal(res.statusCode, 402)
+    t.equal(res.headers['www-authenticate'], undefined)
+    t.same(JSON.parse(res.payload), {
+      error: 'Payment Required',
+      message: 'Winter is coming',
+      statusCode: 402
+    })
+  })
+})
+
 function basicAuthHeader (username, password) {
   return 'Basic ' + Buffer.from(`${username}:${password}`).toString('base64')
 }