Lack of spontaneous pretend play is an early diagnostic indicator of autism spectrum conditions (ASC) along with impaired communication and social interaction. In a previous ISMAR poster [2] we proposed an Augmented Reality (AR) system to encourage pretend play, based on an analogy between imaginative interpretation of physical objects (pretense) and the superimposition of virtual content on the physical world in AR. This paper reports an empirical experiment evaluating that proposal, involving children between the ages of 4 and 7 who have been diagnosed with ASC. Results find significantly more pretend play, and higher engagement, using the AR system by comparison to a non-augmented condition. We also discuss usability issues and design implications for AR systems that aim to support children with ASC and other pervasive developmental disorders.