diff --git a/README.md b/README.md
index 4df874c..4c36569 100644
--- a/README.md
+++ b/README.md
@@ -38,7 +38,7 @@ class MyComponent extends React.Component {
 ```tsx
 type MyComponentProps = {
   prop1: string;
-  prop2: number | undefined;
+  prop2?: number;
 }
 
 type MyComponentState = {
diff --git a/src/transforms/react-js-make-props-and-state-transform.ts b/src/transforms/react-js-make-props-and-state-transform.ts
index 830275f..aacf6f4 100644
--- a/src/transforms/react-js-make-props-and-state-transform.ts
+++ b/src/transforms/react-js-make-props-and-state-transform.ts
@@ -222,16 +222,23 @@ export function reactJSMakePropsAndStateInterfaceTransformFactoryFactory(typeChe
                             console.warn('Bad value for propType', name, 'at', propertyAssignment.getStart());
                             return result;
                         }
-                        const typeValue = getTypeFromReactPropTypeExpression(propertyAssignment.initializer);
 
                         // Ignore children, React types have it
                         if (propertyAssignment.name.getText() === 'children') {
                             return result;
                         }
+
+                        // Ignore children, React types have it
+                        if (propertyAssignment.name.getText() === 'children') {
+                            return result;
+                        }
+
+                        const typeValue = getTypeFromReactPropTypeExpression(propertyAssignment.initializer);
+                        const isOptional = isPropTypeOptional(propertyAssignment.initializer);
                         const propertySignature = ts.createPropertySignature(
                             [],
                             name,
-                            undefined,
+                            isOptional ? ts.createToken(ts.SyntaxKind.QuestionToken): undefined,
                             typeValue,
                             undefined,
                         );
@@ -291,19 +298,16 @@ export function reactJSMakePropsAndStateInterfaceTransformFactoryFactory(typeChe
                 } else {
                     result = ts.createKeywordTypeNode(ts.SyntaxKind.AnyKeyword);
                 }
+                return result;
+            }
 
-                if (!/\.isRequired/.test(text)) {
-                    return makeTypeNodeOptional(result);
-                } else {
-                    return result;
-                }
-
-                function makeTypeNodeOptional(node: ts.TypeNode) {
-                    return ts.createUnionOrIntersectionTypeNode(ts.SyntaxKind.UnionType, [
-                        node,
-                        ts.createKeywordTypeNode(ts.SyntaxKind.UndefinedKeyword)
-                    ]);
-                }
+            /**
+             * Decide if node is optional
+             * @param node React propTypes member node
+             */
+            function isPropTypeOptional(node: ts.PropertyAccessExpression) {
+                const text = node.getText().replace(/React\.PropTypes\./, '');
+                return !/\.isRequired/.test(text)
             }
         };
     };
diff --git a/test/react-js-make-props-and-state-transform/static-proptypes-many-props/output.tsx b/test/react-js-make-props-and-state-transform/static-proptypes-many-props/output.tsx
index 89c5602..5607e07 100644
--- a/test/react-js-make-props-and-state-transform/static-proptypes-many-props/output.tsx
+++ b/test/react-js-make-props-and-state-transform/static-proptypes-many-props/output.tsx
@@ -1,15 +1,15 @@
 import * as React from 'react';
 
 export default class MyComponent extends React.Component<{
-        any: any | undefined;
-        array: any[] | undefined;
-        bool: boolean | undefined;
-        func: ((...args: any[]) => any) | undefined;
-        number: number | undefined;
-        object: object | undefined;
-        string: string | undefined;
-        node: (number | string | JSX.Element) | undefined;
-        element: JSX.Element | undefined;
+        any?: any;
+        array?: any[];
+        bool?: boolean;
+        func?: (...args: any[]) => any;
+        number?: number;
+        object?: object;
+        string?: string;
+        node?: number | string | JSX.Element;
+        element?: JSX.Element;
         anyRequired: any;
         arrayRequired: any[];
         boolRequired: boolean;