diff --git a/Lib/distutils/extension.py b/Lib/distutils/extension.py
--- a/Lib/distutils/extension.py
+++ b/Lib/distutils/extension.py
@@ -106,6 +106,8 @@ class Extension:
         if not (isinstance(sources, list) and
                 all(isinstance(v, str) for v in sources)):
             raise AssertionError("'sources' must be a list of strings")
+        if any(os.path.isabs(v) for v in sources):
+            raise AssertionError("'sources' must contain relative paths")
 
         self.name = name
         self.sources = sources
diff --git a/Lib/distutils/tests/test_build_ext.py b/Lib/distutils/tests/test_build_ext.py
--- a/Lib/distutils/tests/test_build_ext.py
+++ b/Lib/distutils/tests/test_build_ext.py
@@ -41,8 +41,8 @@ class BuildExtTestCase(TempdirManager,
     def test_build_ext(self):
         global ALREADY_TESTED
         copy_xxmodule_c(self.tmp_dir)
-        xx_c = os.path.join(self.tmp_dir, 'xxmodule.c')
-        xx_ext = Extension('xx', [xx_c])
+        os.chdir(self.tmp_dir)
+        xx_ext = Extension('xx', ['xxmodule.c'])
         dist = Distribution({'name': 'xx', 'ext_modules': [xx_ext]})
         dist.package_dir = self.tmp_dir
         cmd = build_ext(dist)
@@ -291,9 +291,9 @@ class BuildExtTestCase(TempdirManager,
 
     def test_get_outputs(self):
         tmp_dir = self.mkdtemp()
-        c_file = os.path.join(tmp_dir, 'foo.c')
-        self.write_file(c_file, 'void PyInit_foo(void) {}\n')
-        ext = Extension('foo', [c_file], optional=False)
+        os.chdir(tmp_dir)
+        self.write_file('foo.c', 'void PyInit_foo(void) {}\n')
+        ext = Extension('foo', ['foo.c'], optional=False)
         dist = Distribution({'name': 'xx',
                              'ext_modules': [ext]})
         cmd = build_ext(dist)
@@ -304,7 +304,7 @@ class BuildExtTestCase(TempdirManager,
         cmd.build_lib = os.path.join(self.tmp_dir, 'build')
         cmd.build_temp = os.path.join(self.tmp_dir, 'tempt')
 
-        # issue #5977 : distutils build_ext.get_outputs
+        # issue #5977: distutils build_ext.get_outputs
         # returns wrong result with --inplace
         other_tmp_dir = os.path.realpath(self.mkdtemp())
         old_wd = os.getcwd()