We evaluated and measured by high performance liquid chromatography the plasma amino acid levels in 11 female patients with anorexia nervosa during a period of significant loss of body weight, compared with 11 healthy age-matched controls. Total amino acid and total branched-chain amino acid levels were similar in both groups, however significantly higher levels of glycine and ornithine were found in anorexia nervosa patients, as well as significantly lower levels of leucine, tyrosine and lysine compared with controls. The glycine/valine and phenylalanine/tyrosine ratios were significantly higher in anorexia nervosa patients than in control subjects. The levels and ratios of other amino acids were within the normal range, with no significant differences between the two groups. Changes in the plasma amino acid profile in anorexia nervosa patients can be explained by chronic severe malnutrition and prolonged stress.