OBJECTIVE: This study was undertaken to obtain detailed measurements of the volume of the brain, using magnetic resonance imaging (MRI), in a carefully selected group of autistic subjects and comparison subjects. METHOD: Twenty-two male autistic subjects and 20 male volunteer comparison subjects were examined with detailed (1.5-mm slices) MRI throughout the entire brain. Total brain, total brain tissue, and total lateral ventricle volumes were measured by using manual tracing and automated techniques. RESULTS: After height and performance IQ were controlled, autistic subjects had significantly greater total brain, total tissue, and total lateral ventricle volumes than comparison subjects. CONCLUSIONS: These findings suggest that male autistic subjects have enlarged brains and that enlargement is a result of both greater brain tissue volume and greater lateral ventricle volume.