Children with Autism Spectrum Disorders (ASD) have communication deficits and difficulties with social interaction. A lack of social behavior can hamper therapeutic interventions and can diminish the ability to learn social skills. Robots have been shown to provoke proactive social behavior in children with ASD. We are developing robot systems capable of acting as catalysts for social behavior in the context of ASD therapy. We present an experiment design for evaluating the effects of a socially assistive robot in a therapeutic setting and results of a pilot experiment with children with ASD interacting with such a robot.