The study examined 75 autistic individuals who met DSM-III-R criteria for autistic disorder at 5 years of age. Diagnosis was based on information elicited using a standardized interview for autism. Two of 75 (2.7%) autistic subjects expressed the fragile X marker (Xq27.3) in 40% of cells counted and had other affected family members. Two additional subjects expressed the fragile X anomaly at low rates (1%). These rates are compared with those reported in other studies. Methodological differences, which may underlie the varying rates reported across studies, are discussed.