Classical citrullinemia generally involves hyperammonemic coma in the first few days of life and leads to neurological sequelae in survivors. We report a case of an elder sister who fell into a hyperammonemic coma on the fifth day after birth. She was successfully treated with intravenous benzoate and hemodialysis, and was subsequently diagnosed with citrullinemia on the basis of biochemical analysis. Two years later, a younger sister was born without prenatal diagnosis. We monitored plasma ammonia and citrulline levels after birth, and again diagnosed her with CTLN1 on the basis of biochemical and DNA analyses. There have been few reports of the prospective treatment of citrullinemia; however, our experience indicates the need for the prospective management and the rapid reduction of ammonia levels to avoid neonatal hyperammonemic coma and subsequent sequelae.