diff --git a/History.md b/History.md
index 099866b..76dbb66 100644
--- a/History.md
+++ b/History.md
@@ -1,6 +1,7 @@
 unreleased
 ==========
 
+  * Accept valid `Vary` header string as `field`
   * Add `vary.append` for low-level string manipulation
 
 0.1.0 / 2014-06-05
diff --git a/README.md b/README.md
index db8e661..82392d0 100644
--- a/README.md
+++ b/README.md
@@ -23,7 +23,8 @@ var vary = require('vary')
 ### vary(res, field)
 
 Adds the given header `field` to the `Vary` response header of `res`.
-This can be a string of a single field or an array of multiple fields.
+This can be a string of a single field, a string of a valid `Vary`
+header, or an array of multiple fields.
 
 This will append the header if not already listed, otherwise leaves
 it listed in the current location.
@@ -36,7 +37,8 @@ vary(res, 'Origin')
 ### vary.append(header, field)
 
 Adds the given header `field` to the `Vary` response header string `header`.
-This can be a string of a single field or an array of multiple fields.
+This can be a string of a single field, a string of a valid `Vary` header,
+or an array of multiple fields.
 
 This will append the header if not already listed, otherwise leaves
 it listed in the current location. The new header string is returned.
diff --git a/index.js b/index.js
index 483d325..1e544e8 100644
--- a/index.js
+++ b/index.js
@@ -37,7 +37,7 @@ function append(header, field) {
 
   // get fields array
   var fields = !Array.isArray(field)
-    ? [String(field)]
+    ? parse(String(field))
     : field;
 
   // assert on invalid fields
@@ -53,7 +53,7 @@ function append(header, field) {
   }
 
   // enumerate current values
-  var vals = header.toLowerCase().split(/ *, */);
+  var vals = parse(header.toLowerCase());
 
   // unspecified vary
   if (fields.indexOf('*') !== -1 || vals.indexOf('*') !== -1) {
@@ -75,6 +75,18 @@ function append(header, field) {
   return header;
 }
 
+/**
+ * Parse a vary header into an array.
+ *
+ * @param {String} header
+ * @return {Array}
+ * @api private
+ */
+
+function parse(header) {
+  return header.trim().split(/ *, */);
+}
+
 /**
  * Mark that a request is varied on a header field.
  *
diff --git a/test/test.js b/test/test.js
index 545201e..00563c3 100644
--- a/test/test.js
+++ b/test/test.js
@@ -35,6 +35,12 @@ describe('vary(res, field)', function () {
         .expect(200, done);
       });
 
+      it('should accept string that is Vary header', function (done) {
+        request(createServer(callVary('foo, bar')))
+        .get('/')
+        .expect(200, done);
+      });
+
       it('should not allow separator ":"', function (done) {
         request(createServer(callVary('invalid:header')))
         .get('/')
@@ -46,12 +52,6 @@ describe('vary(res, field)', function () {
         .get('/')
         .expect(500, /field.*contains.*invalid/, done);
       });
-
-      it('should not allow separator ","', function (done) {
-        request(createServer(callVary('invalid,header')))
-        .get('/')
-        .expect(500, /field.*contains.*invalid/, done);
-      });
     });
   });
 
@@ -166,7 +166,37 @@ describe('vary(res, field)', function () {
     });
   });
 
-  describe('when fields is array', function () {
+  describe('when field is string', function () {
+    it('should set value', function (done) {
+      request(createServer(callVary('Accept')))
+      .get('/')
+      .expect('Vary', 'Accept')
+      .expect(200, done);
+    });
+
+    it('should set value when vary header', function (done) {
+      request(createServer(callVary('Accept, Accept-Encoding')))
+      .get('/')
+      .expect('Vary', 'Accept, Accept-Encoding')
+      .expect(200, done);
+    });
+
+    it('should acept LWS', function (done) {
+      request(createServer(callVary('  Accept     ,     Origin    ')))
+      .get('/')
+      .expect('Vary', 'Accept, Origin')
+      .expect(200, done);
+    });
+
+    it('should handle contained *', function (done) {
+      request(createServer(callVary('Accept,*')))
+      .get('/')
+      .expect('Vary', '*')
+      .expect(200, done);
+    });
+  });
+
+  describe('when field is array', function () {
     it('should set value', function (done) {
       request(createServer(callVary(['Accept', 'Accept-Language'])))
       .get('/')
@@ -225,6 +255,10 @@ describe('vary.append(header, field)', function () {
         vary.append.bind(null, '', 'foo').should.not.throw();
       });
 
+      it('should accept string that is Vary header', function () {
+        vary.append.bind(null, '', 'foo, bar').should.not.throw();
+      });
+
       it('should accept array of string', function () {
         vary.append.bind(null, '', ['foo', 'bar']).should.not.throw();
       });
@@ -236,10 +270,6 @@ describe('vary.append(header, field)', function () {
       it('should not allow separator " "', function () {
         vary.append.bind(null, '', 'invalid header').should.throw(/field.*contains.*invalid/);
       });
-
-      it('should not allow separator ","', function () {
-        vary.append.bind(null, '', 'invalid,header').should.throw(/field.*contains.*invalid/);
-      });
     });
   });
 
@@ -297,7 +327,25 @@ describe('vary.append(header, field)', function () {
     });
   });
 
-  describe('when fields is array', function () {
+  describe('when field is string', function () {
+    it('should set value', function () {
+      vary.append('', 'Accept').should.equal('Accept');
+    });
+
+    it('should set value when vary header', function () {
+      vary.append('', 'Accept, Accept-Encoding').should.equal('Accept, Accept-Encoding');
+    });
+
+    it('should acept LWS', function () {
+      vary.append('', '  Accept     ,     Origin    ').should.equal('Accept, Origin');
+    });
+
+    it('should handle contained *', function () {
+      vary.append('', 'Accept,*').should.equal('*');
+    });
+  });
+
+  describe('when field is array', function () {
     it('should set value', function () {
       vary.append('', ['Accept', 'Accept-Language']).should.equal('Accept, Accept-Language');
     });