There are reports in the literature that suggest slightly impaired average intellectual ability, slightly increased signs of brain irritability, and possibly, a slightly increased susceptibility to late-onset schizophrenia with depressive signs, reproductive anomalies, and varicose veins in heterozygotes for phenylketonuria (PKU). The possible significance of such results for our understanding of genetic liabilities for common disease, or of genetic variability influencing mental performance in the normal range is discussed.