A clinical epidemiologic study of hyperphenylalaninemia.

Thirty-nine cases of hyperphenylalaninemia from four diagnostic-treatment centers in Ohio were investigated in a retrospective cohort analysis. None of the centers is included in the PKU Collaborative Study. Epidemiologic analysis showed that early treatment is effective and that degree of dietary control (within limits) does not affect the probability of retardation or borderline retardation. Regression analysis supported the critical period hypothesis rather than the maximum concentration theory. Exposure to serum phenylalanine of 23--27 mg./dl. corresponds to 50 per cent probability retardation. The clinical epidemiologic approach offers an alternative methodology for designing collaborative studies.