Autistic disorder and catatonia are neuropsychiatric syndromes defined by impairments in social interaction, communication, and restricted, stereotypical motor routines. Assessments of children with these disorders are typically restricted in scope by the patients' limited ability to comprehend directions. The authors performed systematic assessments of dyskinesias on six prepubertal boys with autistic disorder and mental retardation and on one adolescent male with catatonia to determine if this type of information could be routinely obtained. The boys with autistic disorder had more stereotypies and tics, a greater degree of akathisia and hyperactivity, and more compulsions than the adolescent with catatonia. Catatonia was associated with catalepsy and dystonic postures. The authors conclude that the diagnostic accuracy and specificity of neuropsychiatric syndromes may be enhanced by the systematic assessment of the dyskinesias associated with each condition.