A heretofore undescribed disorder manifested by mental retardation, citrullinuria, hypercitrullinemia, and hyperammonemia is reported. Dietary studies show a direct relationship between protein intake and urinary citrulline excretion. Citrulline excretion occurs as an overflow mechanism with plasma citrulline being maintained at about 25 mg/100 ml. Loading studies suggest that the ornithine urea cycle is functioning; high post-absorptive blood ammonia levels indicate that little reserve capacity for urea formation is present.