We report a family with hereditary orotic aciduria heterozygotes. A 3-year-old boy who had been diagnosed as having cerebral palsy and mental retardation presented himself with an increase in excretion of urinary orotic acid. Enzymatic studies revealed that the boy and his healthy mother were hereditary orotic aciduria heterozygote carriers. We can not prove that this pyrimidine disorder caused his neurological symptoms, but his pyrimidine nucleoside supply may have been insufficient in his neonatal period.