A discrete deficiency of hepatic ornithine transcarbamylase (OTC) was found in male patients who were 58, 46 and 17 years old. Each had developed hyperammonemic coma. The mother and a sister of the 17-year-old patient exhibited orotic aciduria either spontaneously or after protein loading, thus demonstrating heterozygosity. A sister of one other patient and a daughter of the third patient showed a smaller orotic aciduria after protein loading. These observations indicate that inherited deficiency of OTC should be included in the differential diagnosis of hyperammonemic states in adult male patients.