diff --git a/usr.bin/diff/diffreg.c b/usr.bin/diff/diffreg.c
--- a/usr.bin/diff/diffreg.c
+++ b/usr.bin/diff/diffreg.c
@@ -1247,7 +1247,8 @@
 				printf(" ");
 		}
 		col = 0;
-		for (j = 0, lastc = '\0'; j < nc; j++, lastc = c) {
+		for (j = 0, lastc = '\0'; j < nc && (hw == 0 || col < hw);
+		    j++, lastc = c) {
 			c = getc(lb);
 			if (flags & D_STRIPCR && c == '\r') {
 				if ((c = getc(lb)) == '\n')
@@ -1274,19 +1275,16 @@
 				if (flags & D_EXPANDTABS) {
 					newcol = ((col / tabsize) + 1) * tabsize;
 					do {
-						if (diff_format == D_SIDEBYSIDE)
-							j++;
 						printf(" ");
-					} while (++col < newcol && j < nc);
+					} while (++col < newcol && col < hw);
 				} else {
 					if (diff_format == D_SIDEBYSIDE) {
-						if ((j + tabsize) > nc) {
-							printf("%*s", nc - j, "");
-							j = col = nc;
+						if ((col + tabsize) > hw) {
+							printf("%*s", hw - col, "");
+							col = hw;
 						} else {
 							printf("\t");
 							col += tabsize - 1;
-							j += tabsize - 1;
 						}
 					} else {
 						printf("\t");
diff --git a/usr.bin/diff/tests/Makefile b/usr.bin/diff/tests/Makefile
--- a/usr.bin/diff/tests/Makefile
+++ b/usr.bin/diff/tests/Makefile
@@ -12,6 +12,8 @@
 	input2.in \
 	input_c1.in \
 	input_c2.in \
+	side_by_side_tabbed_a.in \
+	side_by_side_tabbed_b.in \
 	simple.out \
 	simple_e.out \
 	simple_n.out \
diff --git a/usr.bin/diff/tests/diff_test.sh b/usr.bin/diff/tests/diff_test.sh
--- a/usr.bin/diff/tests/diff_test.sh
+++ b/usr.bin/diff/tests/diff_test.sh
@@ -7,6 +7,7 @@
 atf_test_case ifdef
 atf_test_case group_format
 atf_test_case side_by_side
+atf_test_case side_by_side_tabbed
 atf_test_case brief_format
 atf_test_case b230049
 atf_test_case stripcr_o
@@ -145,6 +146,23 @@
 	    diff -W 65 -y --suppress-common-lines A B
 }
 
+side_by_side_tabbed_body()
+{
+	file_a=$(atf_get_srcdir)/side_by_side_tabbed_a.in
+	file_b=$(atf_get_srcdir)/side_by_side_tabbed_b.in
+
+	atf_check -o save:diffout -s not-exit:0 \
+	    diff -y ${file_a} ${file_b}
+	atf_check -o save:diffout_expanded -s not-exit:0 \
+	    diff -yt ${file_a} ${file_b}
+
+	atf_check -o not-empty grep -Ee 'file A.+file B' diffout
+	atf_check -o not-empty grep -Ee 'file A.+file B' diffout_expanded
+
+	atf_check -o not-empty grep -Ee 'tabs.+tabs' diffout
+	atf_check -o not-empty grep -Ee 'tabs.+tabs' diffout_expanded
+}
+
 brief_format_body()
 {
 	atf_check mkdir A B
@@ -343,6 +361,7 @@
 	atf_add_test_case ifdef
 	atf_add_test_case group_format
 	atf_add_test_case side_by_side
+	atf_add_test_case side_by_side_tabbed
 	atf_add_test_case brief_format
 	atf_add_test_case b230049
 	atf_add_test_case stripcr_o
diff --git a/usr.bin/diff/tests/side_by_side_tabbed_a.in b/usr.bin/diff/tests/side_by_side_tabbed_a.in
new file mode 100644
--- /dev/null
+++ b/usr.bin/diff/tests/side_by_side_tabbed_a.in
@@ -0,0 +1,4 @@
+This
+	is my test file A
+	it has tabs
+Thanks
diff --git a/usr.bin/diff/tests/side_by_side_tabbed_b.in b/usr.bin/diff/tests/side_by_side_tabbed_b.in
new file mode 100644
--- /dev/null
+++ b/usr.bin/diff/tests/side_by_side_tabbed_b.in
@@ -0,0 +1,4 @@
+This
+	is my test file B
+	it has tabs
+Thanks