diff --git a/README.md b/README.md
index 2e4cb3c..1d0c9c7 100644
--- a/README.md
+++ b/README.md
@@ -158,3 +158,28 @@ flatten({
 //   'key3.a': { b: { c: 2 } }
 // }
 ```
+
+### useToJSON
+
+When enabled, `flat` will check for objects with a `toJSON` function and use the result of that function as a value instead of flattening the object. This is particularly useful for complex objects like MongoDB's ObjectID which contain several fields but actually make more sense to present them as a string.
+
+``` javascript
+var flatten = require('flat')
+
+flatten({
+    key1: {
+        keyA: 'valueI',
+        toJSON: function() {
+            return 'JSONvalueI'
+        }
+    },
+    key2: {
+        keyB: 'valueII'
+    }
+}, { useToJSON: true })
+
+// {
+//   'key1': 'JSONvalueI',
+//   'key2.keyB': 'valueII'
+// }
+```
diff --git a/index.js b/index.js
index ca8904a..7e745aa 100644
--- a/index.js
+++ b/index.js
@@ -9,6 +9,7 @@ function flatten (target, opts) {
 
   var delimiter = opts.delimiter || '.'
   var maxDepth = opts.maxDepth
+  var useToJSON = opts.useToJSON
   var output = {}
 
   function step (object, prev, currentDepth) {
@@ -23,16 +24,25 @@ function flatten (target, opts) {
         type === '[object Array]'
       )
 
+      var toJSONExists = false
+      var jsonValue = null
+      if (useToJSON && value === Object(value) && // Check if primitive type
+        Object.prototype.toString.call(value.toJSON) === '[object Function]') {
+        jsonValue = value.toJSON()
+        toJSONExists = true
+      }
+
       var newKey = prev
         ? prev + delimiter + key
         : key
 
-      if (!isarray && !isbuffer && isobject && Object.keys(value).length &&
+      if (!isarray && !isbuffer && isobject && !toJSONExists &&
+        Object.keys(value).length &&
         (!opts.maxDepth || currentDepth < maxDepth)) {
         return step(value, newKey, currentDepth + 1)
       }
 
-      output[newKey] = value
+      output[newKey] = toJSONExists ? jsonValue : value
     })
   }
 
diff --git a/test/test.js b/test/test.js
index 541c9d7..c089e36 100644
--- a/test/test.js
+++ b/test/test.js
@@ -180,6 +180,29 @@ suite('Flatten', function () {
       'hello.0500': 'darkness my old friend'
     })
   })
+
+  test('Use useToJSON', function () {
+    assert.deepEqual(flatten({
+      hello: {
+        world: {
+          again: 'good morning',
+          toJSON: function () {
+            return 'good afternoon'
+          }
+        }
+      },
+      lorem: {
+        ipsum: {
+          dolor: 'good evening'
+        }
+      }
+    }, {
+      useToJSON: true
+    }), {
+      'hello.world': 'good afternoon',
+      'lorem.ipsum.dolor': 'good evening'
+    })
+  })
 })
 
 suite('Unflatten', function () {