diff --git a/src/compiler/core.ts b/src/compiler/core.ts
index cc6a9e6db9a..3a7765094a1 100644
--- a/src/compiler/core.ts
+++ b/src/compiler/core.ts
@@ -900,9 +900,7 @@ namespace ts {
     }
 
     export function fileExtensionIs(path: string, extension: string): boolean {
-        const pathLen = path.length;
-        const extLen = extension.length;
-        return pathLen > extLen && path.substr(pathLen - extLen, extLen) === extension;
+        return stringEndsWith(path, extension);
     }
 
     export function fileExtensionIsAny(path: string, extensions: string[]): boolean {
@@ -915,6 +913,17 @@ namespace ts {
         return false;
     }
 
+    // Should act like String.prototype.startsWith
+    export function stringStartsWith(s: string, start: string): boolean {
+        return s.length > start.length && s.substr(0, start.length) === start;
+    }
+
+    // Should act like String.prototype.endsWith
+    export function stringEndsWith(s: string, end: string): boolean {
+        const sLen = s.length;
+        const endLen = end.length;
+        return sLen > endLen && s.substr(sLen - endLen, endLen) === end;
+    }
 
     // Reserved characters, forces escaping of any non-word (or digit), non-whitespace character.
     // It may be inefficient (we could just match (/[-[\]{}()*+?.,\\^$|#\s]/g), but this is future
diff --git a/src/compiler/program.ts b/src/compiler/program.ts
index 320b628c31a..3f36ea8b1c1 100644
--- a/src/compiler/program.ts
+++ b/src/compiler/program.ts
@@ -112,13 +112,7 @@ namespace ts {
     }
 
     function moduleHasNonRelativeName(moduleName: string): boolean {
-        if (isRootedDiskPath(moduleName)) {
-            return false;
-        }
-
-        const i = moduleName.lastIndexOf("./", 1);
-        const startsWithDotSlashOrDotDotSlash = i === 0 || (i === 1 && moduleName.charCodeAt(0) === CharacterCodes.dot);
-        return !startsWithDotSlashOrDotDotSlash;
+        return !(isRootedDiskPath(moduleName) || isExternalModuleNameRelative(moduleName));
     }
 
     interface ModuleResolutionState {
diff --git a/src/compiler/utilities.ts b/src/compiler/utilities.ts
index 6f3a1d6d813..8dbc052ddcf 100644
--- a/src/compiler/utilities.ts
+++ b/src/compiler/utilities.ts
@@ -1218,7 +1218,23 @@ namespace ts {
     export function isExternalModuleNameRelative(moduleName: string): boolean {
         // TypeScript 1.0 spec (April 2014): 11.2.1
         // An external module name is "relative" if the first term is "." or "..".
-        return moduleName.substr(0, 2) === "./" || moduleName.substr(0, 3) === "../" || moduleName.substr(0, 2) === ".\\" || moduleName.substr(0, 3) === "..\\";
+        if (moduleName.charCodeAt(0) === CharacterCodes.dot) {
+            if (moduleName.length === 1) {
+                return true;
+            }
+            switch (moduleName.charCodeAt(1)) {
+                case CharacterCodes.slash:
+                case CharacterCodes.backslash:
+                    return true;
+                case CharacterCodes.dot:
+                    if (moduleName.length === 2) {
+                        return true;
+                    }
+                    const ch2 = moduleName.charCodeAt(2);
+                    return ch2 === CharacterCodes.slash || ch2 === CharacterCodes.backslash;
+            }
+        }
+        return false;
     }
 
     export function isInstantiatedModule(node: ModuleDeclaration, preserveConstEnums: boolean) {
diff --git a/tests/baselines/reference/relativeModuleWithoutSlash.js b/tests/baselines/reference/relativeModuleWithoutSlash.js
new file mode 100644
index 00000000000..de0a03c7923
--- /dev/null
+++ b/tests/baselines/reference/relativeModuleWithoutSlash.js
@@ -0,0 +1,27 @@
+//// [tests/cases/compiler/relativeModuleWithoutSlash.ts] ////
+
+//// [index.ts]
+export default 0;
+
+//// [index.ts]
+export default 1;
+
+//// [a.ts]
+import parent from "..";
+import here from ".";
+parent + here;
+
+
+//// [index.js]
+"use strict";
+exports.__esModule = true;
+exports["default"] = 0;
+//// [index.js]
+"use strict";
+exports.__esModule = true;
+exports["default"] = 1;
+//// [a.js]
+"use strict";
+var __1 = require("..");
+var _1 = require(".");
+__1["default"] + _1["default"];
diff --git a/tests/baselines/reference/relativeModuleWithoutSlash.symbols b/tests/baselines/reference/relativeModuleWithoutSlash.symbols
new file mode 100644
index 00000000000..280a844f2f7
--- /dev/null
+++ b/tests/baselines/reference/relativeModuleWithoutSlash.symbols
@@ -0,0 +1,17 @@
+=== tests/cases/compiler/index.ts ===
+export default 0;
+No type information for this code.
+No type information for this code.=== tests/cases/compiler/a/index.ts ===
+export default 1;
+No type information for this code.
+No type information for this code.=== tests/cases/compiler/a/a.ts ===
+import parent from "..";
+>parent : Symbol(parent, Decl(a.ts, 0, 6))
+
+import here from ".";
+>here : Symbol(here, Decl(a.ts, 1, 6))
+
+parent + here;
+>parent : Symbol(parent, Decl(a.ts, 0, 6))
+>here : Symbol(here, Decl(a.ts, 1, 6))
+
diff --git a/tests/baselines/reference/relativeModuleWithoutSlash.types b/tests/baselines/reference/relativeModuleWithoutSlash.types
new file mode 100644
index 00000000000..3093aa2e7c6
--- /dev/null
+++ b/tests/baselines/reference/relativeModuleWithoutSlash.types
@@ -0,0 +1,18 @@
+=== tests/cases/compiler/index.ts ===
+export default 0;
+No type information for this code.
+No type information for this code.=== tests/cases/compiler/a/index.ts ===
+export default 1;
+No type information for this code.
+No type information for this code.=== tests/cases/compiler/a/a.ts ===
+import parent from "..";
+>parent : number
+
+import here from ".";
+>here : number
+
+parent + here;
+>parent + here : number
+>parent : number
+>here : number
+
diff --git a/tests/cases/compiler/relativeModuleWithoutSlash.ts b/tests/cases/compiler/relativeModuleWithoutSlash.ts
new file mode 100644
index 00000000000..35ad2ae9c6a
--- /dev/null
+++ b/tests/cases/compiler/relativeModuleWithoutSlash.ts
@@ -0,0 +1,10 @@
+// @Filename: index.ts
+export default 0;
+
+// @Filename: a/index.ts
+export default 1;
+
+// @Filename: a/a.ts
+import parent from "..";
+import here from ".";
+parent + here;