Two infants, one male and one female, with elevated serum ammonia levels, were shown, based on urine organic acid analysis and DNA studies, to have ornithine transcarbamylase (OTC) deficiency. OTC deficiency is one of the most common urea cycle disorders. Hyperammonemia occurred at 3 days of age in the male infant, and at approximately 7 days of age in the female infant. Administration of sodium benzoate and sodium phenylacetate lowered the serum ammonia level effectively in both cases. Other modalities, including peritoneal dialysis and protein restriction, were also important in the control of the serum ammonia level. The mother of the male infant was shown to be a carrier of the OTC gene mutation by allopurinol loading test. The mutation site of the OTC gene for the female infant was identified, but her mother did not have the mutation. OTC deficiency, an incompletely dominant X-linked disorder, is a severe disease even for females and prompt treatment and precise genetic counseling are mandatory.