diff --git a/humanfriendly/__init__.py b/humanfriendly/__init__.py
index 4c0a333..1745cee 100644
--- a/humanfriendly/__init__.py
+++ b/humanfriendly/__init__.py
@@ -353,6 +353,12 @@ def format_number(number, num_decimals=2):
     6,000,000,000
     """
     integer_part, _, decimal_part = str(float(number)).partition('.')
+
+    # Avoid float conversion for integers to prevent bogus output when
+    # str(float()) uses scientific notation (numbers >= 10**16).
+    if type(number) is int:
+        integer_part = str(number)
+        decimal_part = ''
     negative_sign = integer_part.startswith('-')
     reversed_digits = ''.join(reversed(integer_part.lstrip('-')))
     parts = []
diff --git a/humanfriendly/tests.py b/humanfriendly/tests.py
index 72dad99..d92742e 100644
--- a/humanfriendly/tests.py
+++ b/humanfriendly/tests.py
@@ -577,6 +577,10 @@ def test_format_number(self):
         self.assertEqual('1,000,000.42', format_number(1000000.42))
         # Regression test for https://github.com/xolox/python-humanfriendly/issues/40.
         self.assertEqual('-285.67', format_number(-285.67))
+        # Regression test for integer inputs >= 10**16 where float conversion
+        # produces scientific notation that corrupts the output.
+        self.assertEqual('10,000,000,000,000,000', format_number(10 ** 16))
+        self.assertEqual('1,000,000,000,000,000,000', format_number(10 ** 18))
 
     def test_round_number(self):
         """Test :func:`humanfriendly.round_number()`."""