Few studies have examined the occurrence of chromosome abnormalities in a large sample of patients with autism and related pervasive developmental disorders (PDDs). In the present report, the authors examined a consecutive series of 92 children with PDDs (DSM-III-R; 75 males and 17 females). A cytogenetic examination, including growth in folate deficient medium, was performed in all cases. Three patients (3.2%) (two females and one male) showed chromosome abnormalities: deletion of the long arm of chromosome 8; tetrasomy of chromosome 15; and XYY syndrome. Only the subject who had tetrasomy 15 met the criteria for autistic disorder, while the other were diagnosed as suffering from a PDD not otherwise specified (PDDNOS). Another patient showed an abnormal fragile site at Xq27 in three out of 100 cells. However, subsequent molecular studies did not confirm the presence of fragile-X syndrome. These results suggest that chromosome abnormalities are uncommon in traditional autism and may be relatively more common in people with PDDNOS.