diff --git a/Parsedown.php b/Parsedown.php
old mode 100644
new mode 100755
index 382023c..1dde17d
--- a/Parsedown.php
+++ b/Parsedown.php
@@ -90,6 +90,15 @@ class Parsedown
 
     protected $safeMode;
 
+    function setStrictMode($strictMode)
+    {
+        $this->strictMode = (bool) $strictMode;
+
+        return $this;
+    }
+
+    protected $strictMode;
+
     protected $safeLinksWhitelist = array(
         'http://',
         'https://',
@@ -504,36 +513,39 @@ class Parsedown
 
     protected function blockHeader($Line)
     {
-        if (isset($Line['text'][1]))
+        $level = 1;
+
+        while (isset($Line['text'][$level]) and $Line['text'][$level] === '#')
         {
-            $level = 1;
-
-            while (isset($Line['text'][$level]) and $Line['text'][$level] === '#')
-            {
-                $level ++;
-            }
-
-            if ($level > 6)
-            {
-                return;
-            }
-
-            $text = trim($Line['text'], '#');
-            $text = trim($text, ' ');
-
-            $Block = array(
-                'element' => array(
-                    'name' => 'h' . min(6, $level),
-                    'handler' => array(
-                        'function' => 'lineElements',
-                        'argument' => $text,
-                        'destination' => 'elements',
-                    )
-                ),
-            );
-
-            return $Block;
+            $level ++;
         }
+
+        if ($level > 6)
+        {
+            return;
+        }
+
+        $text = trim($Line['text'], '#');
+
+        if ($this->strictMode and isset($text[0]) and $text[0] !== ' ')
+        {
+            return;
+        }
+
+        $text = trim($text, ' ');
+
+        $Block = array(
+            'element' => array(
+                'name' => 'h' . min(6, $level),
+                'handler' => array(
+                    'function' => 'lineElements',
+                    'argument' => $text,
+                    'destination' => 'elements',
+                )
+            ),
+        );
+
+        return $Block;
     }
 
     #
diff --git a/test/ParsedownTest.php b/test/ParsedownTest.php
old mode 100644
new mode 100755
index 01b4e25..bf40317
--- a/test/ParsedownTest.php
+++ b/test/ParsedownTest.php
@@ -51,6 +51,7 @@ class ParsedownTest extends TestCase
         $expectedMarkup = str_replace("\r", "\n", $expectedMarkup);
 
         $this->Parsedown->setSafeMode(substr($test, 0, 3) === 'xss');
+        $this->Parsedown->setStrictMode(substr($test, 0, 6) === 'strict');
 
         $actualMarkup = $this->Parsedown->text($markdown);
 
diff --git a/test/data/atx_heading.html b/test/data/atx_heading.html
index 94c3825..3b09c38 100644
--- a/test/data/atx_heading.html
+++ b/test/data/atx_heading.html
@@ -6,6 +6,8 @@
 <h6>h6</h6>
 <p>####### not a heading</p>
 <h1>closed h1</h1>
-<p>#</p>
+<h1></h1>
+<h2></h2>
 <h1># of levels</h1>
-<h1># of levels #</h1>
\ No newline at end of file
+<h1># of levels #</h1>
+<h1>heading</h1>
\ No newline at end of file
diff --git a/test/data/atx_heading.md b/test/data/atx_heading.md
index 62fdd6c..50f991c 100644
--- a/test/data/atx_heading.md
+++ b/test/data/atx_heading.md
@@ -16,6 +16,10 @@
 
 #
 
+##
+
 # # of levels
 
-# # of levels # #
\ No newline at end of file
+# # of levels # #
+
+#heading
\ No newline at end of file
diff --git a/test/data/strict_atx_heading.html b/test/data/strict_atx_heading.html
new file mode 100644
index 0000000..11cf4df
--- /dev/null
+++ b/test/data/strict_atx_heading.html
@@ -0,0 +1,13 @@
+<h1>h1</h1>
+<h2>h2</h2>
+<h3>h3</h3>
+<h4>h4</h4>
+<h5>h5</h5>
+<h6>h6</h6>
+<p>####### not a heading</p>
+<p>#not a heading</p>
+<h1>closed h1</h1>
+<h1></h1>
+<h2></h2>
+<h1># of levels</h1>
+<h1># of levels #</h1>
\ No newline at end of file
diff --git a/test/data/strict_atx_heading.md b/test/data/strict_atx_heading.md
new file mode 100644
index 0000000..5358731
--- /dev/null
+++ b/test/data/strict_atx_heading.md
@@ -0,0 +1,25 @@
+# h1
+
+## h2
+
+### h3
+
+#### h4
+
+##### h5
+
+###### h6
+
+####### not a heading
+
+#not a heading
+
+# closed h1 #
+
+#
+
+##
+
+# # of levels
+
+# # of levels # #
\ No newline at end of file