diff --git a/.gitignore b/.gitignore
index fe7dd1f..dfda9da 100644
--- a/.gitignore
+++ b/.gitignore
@@ -4,3 +4,4 @@ __pycache__
 peru.egg-info
 build/
 dist/
+.venv/
diff --git a/README.md b/README.md
index e5ba9d5..2a81bb5 100644
--- a/README.md
+++ b/README.md
@@ -332,13 +332,11 @@ control where peru puts things. Flags always take precedence.
 
 - `--file=<file>`: The path to your peru YAML file. By default peru
   looks for `peru.yaml` in the current directory or one of its parents.
-  This setting tells peru to use a specific file. If set, `--sync-dir`
-  must also be set.
+  This setting tells peru to use a specific file.
 - `--sync-dir=<dir>`: The path that all `imports` are interpreted
   relative to. That is, if you import a module to `./`, the contents of
   that module go directly in the sync dir. By default this is the
-  directory containing your `peru.yaml` file. If set, `--file` must also
-  be set.
+  directory containing your `peru.yaml` file.
 - `--state-dir=<dir>`: The directory where peru stashes all of its state
   metadata, and also the parent of the cache dir. By default this is
   `.peru` inside the sync dir. You should not share this directory
diff --git a/peru/runtime.py b/peru/runtime.py
index 431a183..d53f3b4 100644
--- a/peru/runtime.py
+++ b/peru/runtime.py
@@ -61,26 +61,41 @@ async def _init_cache(self):
         self.cache = await cache.Cache(self.cache_dir)
 
     def _set_paths(self, args, env):
-        explicit_peru_file = args['--file']
-        explicit_sync_dir = args['--sync-dir']
         explicit_basename = args['--file-basename']
+        explicit_peru_file = os.path.abspath(
+            args['--file']) if args['--file'] else None
+        explicit_sync_dir = os.path.abspath(
+            args['--sync-dir']) if args['--sync-dir'] else None
+
         if explicit_peru_file and explicit_basename:
             raise CommandLineError(
                 'Cannot use both --file and --file-basename at the same time.')
         if explicit_peru_file and explicit_sync_dir:
             self.peru_file = explicit_peru_file
             self.sync_dir = explicit_sync_dir
-        elif explicit_peru_file or explicit_sync_dir:
-            raise CommandLineError('If the --file or --sync-dir is set, '
-                                   'the other must also be set.')
+        elif explicit_peru_file and not explicit_sync_dir:
+            self.peru_file = explicit_peru_file
+            self.sync_dir = os.path.dirname(self.peru_file)
+        elif not explicit_peru_file and explicit_sync_dir:
+            basename = explicit_basename or parser.DEFAULT_PERU_FILE_NAME
+            self.peru_file = find_project_file(os.getcwd(), basename)
+            self.sync_dir = explicit_sync_dir
         else:
             basename = explicit_basename or parser.DEFAULT_PERU_FILE_NAME
             self.peru_file = find_project_file(os.getcwd(), basename)
             self.sync_dir = os.path.dirname(self.peru_file)
-        self.state_dir = (args['--state-dir']
-                          or os.path.join(self.sync_dir, '.peru'))
-        self.cache_dir = (args['--cache-dir'] or env.get('PERU_CACHE_DIR')
-                          or os.path.join(self.state_dir, 'cache'))
+
+        if args['--state-dir']:
+            self.state_dir = os.path.abspath(args['--state-dir'])
+        else:
+            self.state_dir = os.path.join(self.sync_dir, '.peru')
+
+        if args['--cache-dir']:
+            self.cache_dir = os.path.abspath(args['--cache-dir'])
+        elif env.get('PERU_CACHE_DIR'):
+            self.cache_dir = env.get('PERU_CACHE_DIR')
+        else:
+            self.cache_dir = os.path.join(self.state_dir, 'cache')
 
     def tmp_dir(self):
         dir = tempfile.TemporaryDirectory(dir=self._tmp_root)
diff --git a/tests/test_paths.py b/tests/test_paths.py
index c236d2d..95b1eaa 100644
--- a/tests/test_paths.py
+++ b/tests/test_paths.py
@@ -46,11 +46,6 @@ def test_unmodified_sync(self):
         shared.run_peru_command(['sync'], self.cwd)
         self.assert_success(self.project_dir, self.state_dir, self.cache_dir)
 
-    def test_peru_file_and_sync_dir_must_be_set_together(self):
-        for command in [['--sync-dir=junk', 'sync'], ['--file=junk', 'sync']]:
-            with self.assertRaises(CommandLineError):
-                shared.run_peru_command(command, cwd=self.cwd)
-
     def test_file_and_file_basename_incompatible(self):
         with self.assertRaises(CommandLineError):
             shared.run_peru_command([