Patients with Rett syndrome appear to fulfill the Rendle-Short criteria for the diagnosis of autism, but the pattern of their behavior is qualitatively different from children with autism. Until a biologic marker is identified, diagnosis is based on clinical assessment. In order to standardize this clinical assessment and to provide objective criteria for the evaluation of potential therapeutic modalities, motor and behavioral characteristics of 15 Rett patients were analyzed. The patients with Rett syndrome differed from autistic children in having ataxia, breath-holding, hyperventilation, bruxism, simplicity of stereotypies, and hand apposition. The children with autism demonstrated complex stereotypies and verbal but not motor regression. The more typical features of autism, namely, poor eye contact, lack of sustained interest, speech disturbance, and repetitive truncal rocking motions were poor discriminators between the two groups. (J Child Neurol 1988;3(Suppl): S65-S67).