An 18 yr old mentally and physically retarded boy, suffering from episodes of anorexia, vomiting, coma and convulsion which increased in severity with age, had periodic hyperammonemia, hyperlysinemia and and homocitrullinuria. Blood cell arginase activity of the patient on normal diet was markedly reduced after an oral load of L-lysine. The oral loading tests of L-lysine revealed hyperammonemia, hyperlysinemia, hyperargininemia, hypercitrullinemia and homocitrullinuria. Etiology of metabolic error of the patient was discussed in reference to lysine-urea cycle.