THE FOLLOWING report represents the second known case of citrullinemia. The child was the product of unwed and supposedly unrelated parents. During the first 5½ months of life her foster mother felt that her growth and development were normal except for a severe, persistent diaper rash. At 6 weeks of age she smiled and at 3½ months she began to roll over. Beginning at this time she suffered periods of non-projectile vomiting which lasted for several days at a time. She became less interested in her surroundings, and more irritable. Despite dietary changes her illness progressed and she was admitted at 8 months of age to the Children's Hospital of Philadelphia with hematemises, hematuria, and purpura. Physical examination at the time of admission revealed a responsive but somewhat irritable child who fell in the tenth percentile for both height and weight. The spleen and liver were not palpable. The child