We determined the CGG repeat length and AGG interruptions in the FMR1 gene in normal Chinese subjects and patients with infantile autism and mild mental retardation. Genomic DNA was investigated by PCR and Southern hybridisation for CGG repeat number and PCR with Mnl I restriction analysis for AGG interruption. Both the normal subjects and the patients with autism have 53 CGG repeats in FMR1, and the majority have two interspersed AGG. Our normal Chinese subjects have a similar number of interspersed AGG as other populations. When compared with the normal subjects, the autism patients have less AGG interruptions and a different pattern of AGG distribution. There was a significant difference in the CGG configurations between normal subjects and patients with autism. The latter had less interspersed AGG, as in fragile X patients, but they did not have fragile X. A study on mentally retarded patients with no infantile autism should also be carried out to ascertain whether mental retardation alone may have contributed to such AGG pattern.