diff --git a/src/lexer.c b/src/lexer.c
index 6be47d3..c3c7bf7 100644
--- a/src/lexer.c
+++ b/src/lexer.c
@@ -77,7 +77,8 @@ typedef enum {
     T_cppd_elif,
     T_cppd_else,
     T_cppd_endif,
-    T_cppd_ifdef
+    T_cppd_ifdef,
+    T_cppd_ifndef
 } token_t;
 
 char token_str[MAX_TOKEN_LEN];
@@ -206,6 +207,8 @@ token_t lex_token_internal(bool aliasing)
             return T_cppd_elif;
         if (!strcmp(token_str, "#ifdef"))
             return T_cppd_ifdef;
+        if (!strcmp(token_str, "#ifndef"))
+            return T_cppd_ifndef;
         if (!strcmp(token_str, "#else"))
             return T_cppd_else;
         if (!strcmp(token_str, "#endif"))
diff --git a/src/parser.c b/src/parser.c
index 8ce1c24..fa04232 100644
--- a/src/parser.c
+++ b/src/parser.c
@@ -334,9 +334,9 @@ void cppd_control_flow_skip_lines()
     skip_whitespace();
 }
 
-void check_def(char *alias)
+void check_def(char *alias, bool expected)
 {
-    if (find_alias(alias))
+    if ((find_alias(alias) != NULL) == expected)
         preproc_match = true;
 }
 
@@ -349,7 +349,7 @@ void read_defined_macro()
     lex_ident(T_identifier, lookup_alias);
     lex_expect(T_close_bracket);
 
-    check_def(lookup_alias);
+    check_def(lookup_alias, true);
 }
 
 /* read preprocessor directive at each potential positions: e.g.,
@@ -485,7 +485,20 @@ bool read_preproc_directive()
     if (lex_accept_internal(T_cppd_ifdef, false)) {
         preproc_match = false;
         lex_ident(T_identifier, token);
-        check_def(token);
+        check_def(token, true);
+
+        if (preproc_match) {
+            skip_whitespace();
+            return true;
+        }
+
+        cppd_control_flow_skip_lines();
+        return true;
+    }
+    if (lex_accept_internal(T_cppd_ifndef, false)) {
+        preproc_match = false;
+        lex_ident(T_identifier, token);
+        check_def(token, false);
 
         if (preproc_match) {
             skip_whitespace();
diff --git a/tests/driver.sh b/tests/driver.sh
index 4859a29..31385cd 100755
--- a/tests/driver.sh
+++ b/tests/driver.sh
@@ -458,6 +458,19 @@ int main()
 }
 EOF
 
+# #ifndef...#else...#endif
+try_ 0 << EOF
+#ifndef A
+#define A 0
+#else
+#define A 1
+#endif
+int main()
+{
+    return A;
+}
+EOF
+
 # #if defined(...) ... #elif defined(...) ... #else ... #endif
 try_ 0 << EOF
 #define A 0