Serum selenium levels in 73 patients with phenylketonuria were significantly lower than in controls. The phenylketonuric and hyperphenylalaninaemic individuals taking the non-supplemented amino acid mixture generally had lower levels: 36% were below the normal range as defined by our laboratory, compared with 19% in the supplemented group. The low levels were present even in those on diet, who had a greater phenylalanine tolerance--that is, a tolerance for more than 9 x 50 mg phenylalanine exchanges per day, in other words a higher intake of natural protein. Individuals on long-term synthetic diets may be at risk for selenium deficiency even on selenium supplements. In areas where the soil may be low in selenium, the deficiency may be aggravated. Long-term low levels may impair health but the required amount of selenium supplementation remains uncertain.