We undertook a retrospective etiological study of all children referred for evaluation of pervasive developmental disorder (PDD). We identified 91 children who met the DSM III-R criteria for PDD. Fifty-two were diagnosed with autistic disorder (AD), and 39 with PDD-not otherwise specified (PDD-NOS). Seven families (8.2%) had more than one affected sib. The overall recurrence rate was 7.1%. Six families had a positive history of PDD in more distant relatives. An excess of developmental problems were identified on the maternal side (seven families, vs two families on the paternal side). Affected children had head circumferences above the mean when compared with standardized growth curves. A recognizable syndrome or genetic disorder was identified in 14 children (15.4%), of which 8 children (9%) were thought to be causative of PDD (5 children with Rett syndrome, 2 with fragile X syndrome, and 1 with velocardiofacial syndrome [VCFS]). Six others had a recognized genetic, cytogenetic, or metabolic disorder believed to be unrelated to the PDD diagnosis. Given the relatively high yield of genetic diagnoses in this population, we believe that children with PDD-NOS or AD should have a detailed evaluation by a clinical geneticist or pediatrician trained in dysmorphology. Chromosome anomalies, fragile X, and other recognizable disorders, including VCFS, need to be excluded. The value of general screening for an inborn error of metabolism in all children with PDD is not certain. In light of the relatively high recurrence of PDD in families, genetic counseling is recommended.