The prevalence of the 985A-to-G mutation in the medium-chain acyl-CoA dehydrogenase (MCAD) gene among Japanese patients with sudden infant death syndrome, Reye syndrome, unknown fatty acid oxidation disorders and severe motor and intellectual disabilities was studied using the PCR/Nco-I method for molecular diagnosis. A frequency study of this common mutation was also conducted on blood samples and left over Guthrie cards from 329 healthy newborns in Japan. Neither heterozygotes nor homozygotes for the 985A-to-G mutation were identified among both patients and controls. The result of the present study accord with previous reports that MCAD deficiency is a common disorder in Caucasians, but quite rare among Japanese. Therefore, newborn mass-screening for MCAD deficiency using this method will not be practical in Japan. However, it still seems necessary to investigate a child with fatty acid oxidation disorder for the presence of MCAD deficiency, using both biochemical and molecular genetic methods.