diff --git a/lib/Request.php b/lib/Request.php
index 99a13d2..c0ab31c 100644
--- a/lib/Request.php
+++ b/lib/Request.php
@@ -110,10 +110,17 @@ public function setAbsoluteUrl(string $url)
     public function getAbsoluteUrl(): string
     {
         if (!$this->absoluteUrl) {
-            // Guessing we're a http endpoint.
-            $this->absoluteUrl = 'http://'.
-                ($this->getHeader('Host') ?? 'localhost').
-                $this->getUrl();
+            $url = $this->getUrl();
+            if (parse_url($url, PHP_URL_SCHEME)) {
+                // It's already an absolute URL
+                $this->absoluteUrl = $url;
+            } else {
+                $host = $this->getHeader('Host')
+                    ?? parse_url($url, PHP_URL_HOST)
+                    ?? 'localhost';
+                // Guessing we're a http endpoint.
+                $this->absoluteUrl = "http://$host$url";
+            }
         }
 
         return $this->absoluteUrl;
diff --git a/tests/HTTP/RequestTest.php b/tests/HTTP/RequestTest.php
index a810900..e432a78 100644
--- a/tests/HTTP/RequestTest.php
+++ b/tests/HTTP/RequestTest.php
@@ -134,4 +134,18 @@ public function testToStringAuthorization()
                   .'foo';
         $this->assertEquals($expected, (string) $request);
     }
+
+    public function testAbsoluteUrlHttp(): void
+    {
+        $request = new Request('GET', 'http://example.com/foo/bar?a=b&c=d');
+        self::assertEquals('http://example.com/foo/bar?a=b&c=d', $request->getAbsoluteUrl());
+    }
+
+    public function testAbsoluteUrlHttpHostPrevalence(): void
+    {
+        $request = new Request('GET', 'http://example.com/foo/bar?a=b&c=d', [
+            'Host' => 'example.org',
+        ]);
+        self::assertEquals('http://example.com/foo/bar?a=b&c=d', $request->getAbsoluteUrl());
+    }
 }