diff --git a/pipreqs/pipreqs.py b/pipreqs/pipreqs.py
index 431764a..fb93beb 100755
--- a/pipreqs/pipreqs.py
+++ b/pipreqs/pipreqs.py
@@ -18,6 +18,7 @@ Options:
     --savepath <file>     Save the list of requirements in the given file
     --print               Output the list of requirements in the standard output
     --force               Overwrite existing requirements.txt
+    --diff <file>         Compare modules in requirements.txt to project imports.
 """
 from __future__ import print_function, absolute_import
 import os
@@ -239,7 +240,7 @@ def parse_requirements(file_):
         tuple: The contents of the file, excluding comments.
     """
     modules = []
-    delim = ["<", ">", "=", "!", "~"]   # https://www.python.org/dev/peps/pep-0508/#complete-grammar
+    delim = ["<", ">", "=", "!", "~"]  # https://www.python.org/dev/peps/pep-0508/#complete-grammar
 
     try:
         with open(file_, "r") as f:
@@ -252,7 +253,7 @@ def parse_requirements(file_):
     data = [x for x in data if x[0].isalpha()]
 
     for x in data:
-        if not any([y in x for y in delim]): # Check for modules w/o a specifier.
+        if not any([y in x for y in delim]):  # Check for modules w/o a specifier.
             modules.append({"name": x, "version": None})
         for y in x:
             if y in delim:
@@ -289,6 +290,14 @@ def compare_modules(file_, imports):
     return modules_not_imported
 
 
+def diff(file_, imports):
+    """Display the difference between modules in file a and imported modules."""
+    modules_not_imported = compare_modules(file_, imports)
+
+    logging.info("The following modules are in {} but do not seem to be imported: "
+                 "{}".format(file_, ", ".join(x for x in modules_not_imported)))
+
+
 def init(args):
     encoding = args.get('--encoding')
     extra_ignore_dirs = args.get('--ignore')
@@ -326,6 +335,10 @@ def init(args):
     path = (args["--savepath"] if args["--savepath"] else
             os.path.join(args['<path>'], "requirements.txt"))
 
+    if args["--diff"]:
+        diff(args["--diff"], imports)
+        return
+
     if not args["--print"] and not args["--savepath"] and not args["--force"] and os.path.exists(path):
         logging.warning("Requirements.txt already exists, "
                         "use --force to overwrite it")