diff --git a/pre_commit/commands.py b/pre_commit/commands.py
index ac9d4487..defd4199 100644
--- a/pre_commit/commands.py
+++ b/pre_commit/commands.py
@@ -30,6 +30,9 @@ COLS = int(subprocess.Popen(['tput', 'cols'], stdout=subprocess.PIPE).communicat
 
 PASS_FAIL_LENGTH = 6
 
+# Grabbed from `git help status`
+CONFLICTING_GIT_STATUSES = set(('DD', 'AU', 'UD', 'UA', 'DU', 'AA', 'UU'))
+
 
 def install(runner):
     """Install the pre-commit hooks."""
@@ -229,11 +232,24 @@ def _run_hook(runner, hook_id, args, write):
         return 1
 
 
+def _has_unmerged_paths(runner):
+    _, stdout, _ = runner.cmd_runner.run(
+        ['git', 'status', '--short'], retcode=None,
+    )
+    codes = set(line[:2] for line in stdout.splitlines())
+    return codes & CONFLICTING_GIT_STATUSES > set()
+
+
 def run(runner, args, write=sys.stdout.write):
     # Set up our logging handler
     logger.addHandler(LoggingHandler(args.color, write=write))
     logger.setLevel(logging.INFO)
 
+    # Check if we have unresolved merge conflict files and fail fast.
+    if _has_unmerged_paths(runner):
+        logger.error('Unmerged files.  Resolve before committing.')
+        return 1
+
     if args.no_stash or args.all_files:
         ctx = noop_context()
     else:
diff --git a/tests/commands_test.py b/tests/commands_test.py
index 40530461..37fe3aea 100644
--- a/tests/commands_test.py
+++ b/tests/commands_test.py
@@ -277,6 +277,17 @@ def test_no_stash(repo_with_passing_hook, no_stash, all_files, expect_stash):
         assert warning_msg not in printed
 
 
+@pytest.mark.parametrize(
+    ('mode', 'expected'),
+    [(status, True) for status in commands.CONFLICTING_GIT_STATUSES] +
+    [(' A', False), (' D', False), (' M', False)]
+)
+def test_has_unmerged_paths(mode, expected):
+    mock_runner = mock.Mock()
+    mock_runner.cmd_runner.run.return_value = (1, mode + ' foo', '')
+    assert commands._has_unmerged_paths(mock_runner) is expected
+
+
 @pytest.yield_fixture
 def in_merge_conflict(repo_with_passing_hook):
     local['git']['add', C.CONFIG_FILE]()
@@ -298,6 +309,12 @@ def in_merge_conflict(repo_with_passing_hook):
 
 
 def test_merge_conflict(in_merge_conflict):
+    ret, printed = _do_run(in_merge_conflict, _get_opts())
+    assert ret == 1
+    assert 'Unmerged files.  Resolve before committing.' in printed
+
+
+def test_merge_conflict_modified(in_merge_conflict):
     # Touch another file so we have unstaged non-conflicting things
     assert os.path.exists('dummy')
     with open('dummy', 'w') as dummy_file:
@@ -305,4 +322,4 @@ def test_merge_conflict(in_merge_conflict):
 
     ret, printed = _do_run(in_merge_conflict, _get_opts())
     assert ret == 1
-    assert 'Resolve merge conflicts before committing' in printed
+    assert 'Unmerged files.  Resolve before committing.' in printed
diff --git a/tests/staged_files_only_test.py b/tests/staged_files_only_test.py
index adb0b015..e3533733 100644
--- a/tests/staged_files_only_test.py
+++ b/tests/staged_files_only_test.py
@@ -227,7 +227,7 @@ def fake_logging_handler():
             self.logs = []
 
         def emit(self, record):
-            self.logs.append(record)
+            self.logs.append(record)  # pragma: no cover (only hit in failure)
 
     pre_commit_logger = logging.getLogger('pre_commit')
     original_level = pre_commit_logger.getEffectiveLevel()