The Autism Behavior Checklist (ABC), an assessment instrument for autistic individuals, was evaluated in a group of 157 subjects, 94 clinically autistic and 63 nonautistic. The two groups differed significantly in ratings of pathology. Both false positive and false negative diagnostic classifications were made when the results of the checklist were compared with clinical diagnosis. Effects of developmental level and age were observed. The ABC appears to have merit as a screening instrument, though results of the checklist alone cannot be taken as establishing a diagnosis of autism. Important issues of reliability and validity remain to be addressed.