diff --git a/src/aphront/configuration/AphrontApplicationConfiguration.php b/src/aphront/configuration/AphrontApplicationConfiguration.php
--- a/src/aphront/configuration/AphrontApplicationConfiguration.php
+++ b/src/aphront/configuration/AphrontApplicationConfiguration.php
@@ -771,12 +771,21 @@
       );
     }
 
+    $raw_input = @file_get_contents('php://input');
+    if ($raw_input !== false) {
+      $base64_input = base64_encode($raw_input);
+    } else {
+      $base64_input = null;
+    }
+
     $result = array(
       'path' => $path,
       'params' => $params,
       'user' => idx($_SERVER, 'PHP_AUTH_USER'),
       'pass' => idx($_SERVER, 'PHP_AUTH_PW'),
 
+      'raw.base64' => $base64_input,
+
       // This just makes sure that the response compresses well, so reasonable
       // algorithms should want to gzip or deflate it.
       'filler' => str_repeat('Q', 1024 * 16),
diff --git a/src/aphront/requeststream/AphrontRequestStream.php b/src/aphront/requeststream/AphrontRequestStream.php
--- a/src/aphront/requeststream/AphrontRequestStream.php
+++ b/src/aphront/requeststream/AphrontRequestStream.php
@@ -89,4 +89,24 @@
     return $stream;
   }
 
+  public static function supportsGzip() {
+    if (!function_exists('gzencode') || !function_exists('gzdecode')) {
+      return false;
+    }
+
+    $has_zlib = false;
+
+    // NOTE: At least locally, this returns "zlib.*", which is not terribly
+    // reassuring. We care about "zlib.inflate".
+
+    $filters = stream_get_filters();
+    foreach ($filters as $filter) {
+      if (preg_match('/^zlib\\./', $filter)) {
+        $has_zlib = true;
+      }
+    }
+
+    return $has_zlib;
+  }
+
 }
diff --git a/src/applications/config/check/PhabricatorWebServerSetupCheck.php b/src/applications/config/check/PhabricatorWebServerSetupCheck.php
--- a/src/applications/config/check/PhabricatorWebServerSetupCheck.php
+++ b/src/applications/config/check/PhabricatorWebServerSetupCheck.php
@@ -50,6 +50,20 @@
         new PhutilOpaqueEnvelope($expect_pass))
       ->setTimeout(5);
 
+    if (AphrontRequestStream::supportsGzip()) {
+      $gzip_uncompressed = str_repeat('Quack! ', 128);
+      $gzip_compressed = gzencode($gzip_uncompressed);
+
+      $gzip_future = id(new HTTPSFuture($base_uri))
+        ->addHeader('X-Phabricator-SelfCheck', 1)
+        ->addHeader('Content-Encoding', 'gzip')
+        ->setTimeout(5)
+        ->setData($gzip_compressed);
+
+    } else {
+      $gzip_future = null;
+    }
+
     // Make a request to the metadata service available on EC2 instances,
     // to test if we're running on a T2 instance in AWS so we can warn that
     // this is a bad idea. Outside of AWS, this request will just fail.
@@ -61,12 +75,16 @@
       $self_future,
       $ec2_future,
     );
+
+    if ($gzip_future) {
+      $futures[] = $gzip_future;
+    }
+
     $futures = new FutureIterator($futures);
     foreach ($futures as $future) {
       // Just resolve the futures here.
     }
 
-
     try {
       list($body) = $ec2_future->resolvex();
       $body = trim($body);
@@ -259,6 +277,107 @@
         ->setMessage($message);
     }
 
+    if ($gzip_future) {
+      $this->checkGzipResponse(
+        $gzip_future,
+        $gzip_uncompressed,
+        $gzip_compressed);
+    }
+  }
+
+  private function checkGzipResponse(
+    Future $future,
+    $uncompressed,
+    $compressed) {
+
+    try {
+      list($body, $headers) = $future->resolvex();
+    } catch (Exception $ex) {
+      return;
+    }
+
+    try {
+      $structure = phutil_json_decode(trim($body));
+    } catch (Exception $ex) {
+      return;
+    }
+
+    $raw_body = idx($structure, 'raw.base64');
+    $raw_body = base64_decode($raw_body);
+
+    // The server received the exact compressed bytes we expected it to, so
+    // everything is working great.
+    if ($raw_body === $compressed) {
+      return;
+    }
+
+    // If the server received a prefix of the raw uncompressed string, it
+    // is almost certainly configured to decompress responses inline. Guide
+    // users to this problem narrowly.
+
+    // Otherwise, something is wrong but we don't have much of a clue what.
+
+    $message = array();
+    $message[] = pht(
+      'Phabricator sent itself a test request that was compressed with '.
+      '"Content-Encoding: gzip", but received different bytes than it '.
+      'sent.');
+
+    $prefix_len = min(strlen($raw_body), strlen($uncompressed));
+    if ($prefix_len > 16 && !strncmp($raw_body, $uncompressed, $prefix_len)) {
+      $message[] = pht(
+        'The request body that the server received had already been '.
+        'decompressed. This strongly suggests your webserver is configured '.
+        'to decompress requests inline, before they reach PHP.');
+      $message[] = pht(
+        'If you are using Apache, your server may be configured with '.
+        '"SetInputFilter DEFLATE". This directive destructively mangles '.
+        'requests and emits them with "Content-Length" and '.
+        '"Content-Encoding" headers that no longer match the data in the '.
+        'request body.');
+    } else {
+      $message[] = pht(
+        'This suggests your webserver is configured to decompress or mangle '.
+        'compressed requests.');
+
+      $message[] = pht(
+        'The request body Phabricator sent began:');
+      $message[] = $this->snipBytes($compressed);
+
+      $message[] = pht(
+        'The request body Phabricator received began:');
+      $message[] = $this->snipBytes($raw_body);
+    }
+
+    $message[] = pht(
+      'Identify the component in your webserver configuration which is '.
+      'decompressing or mangling requests and disable it. Phabricator '.
+      'will not work properly until you do.');
+
+    $message = phutil_implode_html("\n\n", $message);
+
+    $this->newIssue('webserver.accept-gzip')
+      ->setName(pht('Compressed Requests Not Received Properly'))
+      ->setSummary(
+        pht(
+          'Your webserver is not handling compressed request bodies '.
+          'properly.'))
+      ->setMessage($message);
+  }
+
+  private function snipBytes($raw) {
+    if (!strlen($raw)) {
+      $display = pht('<empty>');
+    } else {
+      $snip = substr($raw, 0, 24);
+      $display = phutil_loggable_string($snip);
+
+      if (strlen($snip) < strlen($raw)) {
+        $display .= '...';
+      }
+    }
+
+    return phutil_tag('tt', array(), $display);
   }
 
 }