diff --git a/src/transaction.js b/src/transaction.js
index 6bbf613..c195da3 100644
--- a/src/transaction.js
+++ b/src/transaction.js
@@ -34,7 +34,7 @@ function Transaction() {
  *
  * Note that this method does not sign the created input.
  */
-Transaction.prototype.addInput = function(tx, outIndex) {
+Transaction.prototype.addInput = function(tx, index) {
   var hash
 
   if (typeof tx === 'string') {
@@ -50,13 +50,15 @@ Transaction.prototype.addInput = function(tx, outIndex) {
 
   }
 
-  this.ins.push(new TransactionIn({
+  assert.equal(typeof index, 'number', 'Expected number index, got ' + index)
+
+  return (this.ins.push(new TransactionIn({
     outpoint: {
       hash: hash,
-      index: outIndex
+      index: index
     },
     script: Script.EMPTY
-  }))
+  })) - 1)
 }
 
 /**
@@ -81,10 +83,10 @@ Transaction.prototype.addOutput = function(scriptPubKey, value) {
     scriptPubKey = address.toOutputScript()
   }
 
-  this.outs.push(new TransactionOut({
+  return (this.outs.push(new TransactionOut({
     script: scriptPubKey,
     value: value,
-  }))
+  })) - 1)
 }
 
 Transaction.prototype.toBuffer = function () {
diff --git a/test/transaction.js b/test/transaction.js
index bcb3179..b2c8b92 100644
--- a/test/transaction.js
+++ b/test/transaction.js
@@ -107,6 +107,11 @@ describe('Transaction', function() {
         verifyTransactionIn()
       })
 
+      it('returns an index', function() {
+        assert.equal(tx.addInput(prevTx, 0), 0)
+        assert.equal(tx.addInput(prevTx, 0), 1)
+      })
+
       function verifyTransactionIn() {
         assert.equal(tx.ins.length, 1)
 
@@ -142,6 +147,13 @@ describe('Transaction', function() {
         verifyTransactionOut()
       })
 
+      it('returns an index', function() {
+        var dest = '15mMHKL96tWAUtqF3tbVf99Z8arcmnJrr3'
+
+        assert.equal(tx.addOutput(dest, 40000), 0)
+        assert.equal(tx.addOutput(dest, 40000), 1)
+      })
+
       function verifyTransactionOut() {
         assert.equal(tx.outs.length, 1)