The classification of autism spectrum disorders (ASDs) is a topic of debate among clinicians and researchers with many questioning the validity of the distinction among subtypes. This manuscript examines the validity of three ASD subtypes (Autism, Asperger's, and PDDNOS) by reviewing 22 studies published between 1994 and 2006. We reviewed studies that examined differences between the subtypes in terms of clinical and demographic characteristics, neuropsychological profiles, comorbidity, and prognosis. Results largely did not support differences between autism and Asperger's disorder based on current diagnostic criteria. Overall, the most salient group differences were noted when samples were categorized on IQ. Drawing definitive conclusions is difficult due to the inconsistent application of diagnostic criteria and circularity in methods.