diff --git a/Documentation/git-check-ignore.txt b/Documentation/git-check-ignore.txt
index d2df487aa2..ee2e091704 100644
--- a/Documentation/git-check-ignore.txt
+++ b/Documentation/git-check-ignore.txt
@@ -45,6 +45,13 @@ OPTIONS
 	not be possible to distinguish between paths which match a
 	pattern and those which don't.
 
+--no-index::
+	Don't look in the index when undertaking the checks. This can
+	be used to debug why a path became tracked by e.g. `git add .`
+	and was not ignored by the rules as expected by the user or when
+	developing patterns including negation to match a path previously
+	added with `git add -f`.
+
 OUTPUT
 ------
 
diff --git a/builtin/check-ignore.c b/builtin/check-ignore.c
index e2a1cef684..594463a11b 100644
--- a/builtin/check-ignore.c
+++ b/builtin/check-ignore.c
@@ -5,7 +5,7 @@
 #include "pathspec.h"
 #include "parse-options.h"
 
-static int quiet, verbose, stdin_paths, show_non_matching;
+static int quiet, verbose, stdin_paths, show_non_matching, no_index;
 static const char * const check_ignore_usage[] = {
 "git check-ignore [options] pathname...",
 "git check-ignore [options] --stdin < <list-of-paths>",
@@ -24,6 +24,8 @@ static const struct option check_ignore_options[] = {
 		 N_("terminate input and output records by a NUL character")),
 	OPT_BOOL('n', "non-matching", &show_non_matching,
 		 N_("show non-matching input paths")),
+	OPT_BOOL(0, "no-index", &no_index,
+		 N_("ignore index when checking")),
 	OPT_END()
 };
 
@@ -166,7 +168,7 @@ int cmd_check_ignore(int argc, const char **argv, const char *prefix)
 		die(_("--non-matching is only valid with --verbose"));
 
 	/* read_cache() is only necessary so we can watch out for submodules. */
-	if (read_cache() < 0)
+	if (!no_index && read_cache() < 0)
 		die(_("index file corrupt"));
 
 	memset(&dir, 0, sizeof(dir));
diff --git a/t/t0008-ignores.sh b/t/t0008-ignores.sh
index 96f40fedfb..181513ab4f 100755
--- a/t/t0008-ignores.sh
+++ b/t/t0008-ignores.sh
@@ -66,11 +66,11 @@ test_check_ignore () {
 
 	init_vars &&
 	rm -f "$HOME/stdout" "$HOME/stderr" "$HOME/cmd" &&
-	echo git $global_args check-ignore $quiet_opt $verbose_opt $non_matching_opt $args \
+	echo git $global_args check-ignore $quiet_opt $verbose_opt $non_matching_opt $no_index_opt $args \
 		>"$HOME/cmd" &&
 	echo "$expect_code" >"$HOME/expected-exit-code" &&
 	test_expect_code "$expect_code" \
-		git $global_args check-ignore $quiet_opt $verbose_opt $non_matching_opt $args \
+		git $global_args check-ignore $quiet_opt $verbose_opt $non_matching_opt $no_index_opt $args \
 		>"$HOME/stdout" 2>"$HOME/stderr" &&
 	test_cmp "$HOME/expected-stdout" "$HOME/stdout" &&
 	stderr_empty_on_success "$expect_code"
@@ -87,6 +87,9 @@ test_check_ignore () {
 # check-ignore --verbose output is the same as normal output except
 # for the extra first column.
 #
+# A parameter is used to determine if the tests are run with the
+# normal case (using the index), or with the --no-index option.
+#
 # Arguments:
 #   - (optional) prereqs for this test, e.g. 'SYMLINKS'
 #   - test name
@@ -94,19 +97,26 @@ test_check_ignore () {
 #     from the other verbosity modes is automatically inferred
 #     from this value)
 #   - code to run (should invoke test_check_ignore)
-test_expect_success_multi () {
+#   - index option: --index or --no-index
+test_expect_success_multiple () {
 	prereq=
-	if test $# -eq 4
+	if test $# -eq 5
 	then
 		prereq=$1
 		shift
 	fi
+	if test "$4" = "--index"
+	then
+		no_index_opt=
+	else
+		no_index_opt=$4
+	fi
 	testname="$1" expect_all="$2" code="$3"
 
 	expect_verbose=$( echo "$expect_all" | grep -v '^::	' )
 	expect=$( echo "$expect_verbose" | sed -e 's/.*	//' )
 
-	test_expect_success $prereq "$testname" '
+	test_expect_success $prereq "$testname${no_index_opt:+ with $no_index_opt}" '
 		expect "$expect" &&
 		eval "$code"
 	'
@@ -116,7 +126,8 @@ test_expect_success_multi () {
 	then
 		for quiet_opt in '-q' '--quiet'
 		do
-			test_expect_success $prereq "$testname${quiet_opt:+ with $quiet_opt}" "
+			opts="${no_index_opt:+$no_index_opt }$quiet_opt"
+			test_expect_success $prereq "$testname${opts:+ with $opts}" "
 			expect '' &&
 			$code
 		"
@@ -126,7 +137,7 @@ test_expect_success_multi () {
 
 	for verbose_opt in '-v' '--verbose'
 	do
-		for non_matching_opt in '' ' -n' ' --non-matching'
+		for non_matching_opt in '' '-n' '--non-matching'
 		do
 			if test -n "$non_matching_opt"
 			then
@@ -139,12 +150,21 @@ test_expect_success_multi () {
 				expect '$my_expect' &&
 				$code
 			"
-			opts="$verbose_opt$non_matching_opt"
+			opts="${no_index_opt:+$no_index_opt }$verbose_opt${non_matching_opt:+ $non_matching_opt}"
 			test_expect_success $prereq "$testname${opts:+ with $opts}" "$test_code"
 		done
 	done
 	verbose_opt=
 	non_matching_opt=
+	no_index_opt=
+}
+
+test_expect_success_multi () {
+	test_expect_success_multiple "$@" "--index"
+}
+
+test_expect_success_no_index_multi () {
+	test_expect_success_multiple "$@" "--no-index"
 }
 
 test_expect_success 'setup' '
@@ -288,7 +308,7 @@ test_expect_success_multi 'needs work tree' '' '
 
 # First make sure that the presence of a file in the working tree
 # does not impact results, but that the presence of a file in the
-# index does.
+# index does unless the --no-index option is used.
 
 for subdir in '' 'a/'
 do
@@ -303,27 +323,61 @@ do
 		"::	${subdir}non-existent" \
 		"test_check_ignore '${subdir}non-existent' 1"
 
+	test_expect_success_no_index_multi "non-existent file $where not ignored" \
+		"::	${subdir}non-existent" \
+		"test_check_ignore '${subdir}non-existent' 1"
+
 	test_expect_success_multi "non-existent file $where ignored" \
 		".gitignore:1:one	${subdir}one" \
 		"test_check_ignore '${subdir}one'"
 
+	test_expect_success_no_index_multi "non-existent file $where ignored" \
+		".gitignore:1:one	${subdir}one" \
+		"test_check_ignore '${subdir}one'"
+
 	test_expect_success_multi "existing untracked file $where not ignored" \
 		"::	${subdir}not-ignored" \
 		"test_check_ignore '${subdir}not-ignored' 1"
 
+	test_expect_success_no_index_multi "existing untracked file $where not ignored" \
+		"::	${subdir}not-ignored" \
+		"test_check_ignore '${subdir}not-ignored' 1"
+
 	test_expect_success_multi "existing tracked file $where not ignored" \
 		"::	${subdir}ignored-but-in-index" \
 		"test_check_ignore '${subdir}ignored-but-in-index' 1"
 
+	test_expect_success_no_index_multi "existing tracked file $where shown as ignored" \
+		".gitignore:2:ignored-*	${subdir}ignored-but-in-index" \
+		"test_check_ignore '${subdir}ignored-but-in-index'"
+
 	test_expect_success_multi "existing untracked file $where ignored" \
 		".gitignore:2:ignored-*	${subdir}ignored-and-untracked" \
 		"test_check_ignore '${subdir}ignored-and-untracked'"
 
+	test_expect_success_no_index_multi "existing untracked file $where ignored" \
+		".gitignore:2:ignored-*	${subdir}ignored-and-untracked" \
+		"test_check_ignore '${subdir}ignored-and-untracked'"
+
 	test_expect_success_multi "mix of file types $where" \
 "::	${subdir}non-existent
 .gitignore:1:one	${subdir}one
 ::	${subdir}not-ignored
 ::	${subdir}ignored-but-in-index
+.gitignore:2:ignored-*	${subdir}ignored-and-untracked" \
+		"test_check_ignore '
+			${subdir}non-existent
+			${subdir}one
+			${subdir}not-ignored
+			${subdir}ignored-but-in-index
+			${subdir}ignored-and-untracked'
+		"
+
+	test_expect_success_no_index_multi "mix of file types $where" \
+"::	${subdir}non-existent
+.gitignore:1:one	${subdir}one
+::	${subdir}not-ignored
+.gitignore:2:ignored-*	${subdir}ignored-but-in-index
 .gitignore:2:ignored-*	${subdir}ignored-and-untracked" \
 		"test_check_ignore '
 			${subdir}non-existent