A 9-year-old girl with hereditary dibasicaminoaciduria has been studied for three years. Initially, clinical features were: growth failure; anorexia and aversion to protein, spontaneous daily protein intake averaging only 10 gm; fasting and postprandial venous hyperammonemia; subnormal plasma concentrations of lysine, arginine, ornithine, and citrulline, with generalized hypermonobasic-aminoacidemia; abnormally high renal clearances of lysine, arginine, and ornithine; and intestinal malabsorption of lysine and arginine. Intestinal absorption of citrulline, a precursor of arginine and ornithine, was normal. The patient was observed during four sequential 6-month periods as follows: no treatment (Period I); dietary supplement of arginine and lysine (Period II); dietary supplement of citrulline and lysine (Period III); no treatment (Period IV). During Periods II and III growth rate increased 3- to 4-fold, spontaneous protein intake increased 2- to 3-fold, and abnormalities in blood NH₃ and the plasma aminogram were partially corrected. In most respects the citrulline plus lysine supplement was more beneficial than that of arginine plus lysine.