The inheritance of alcoholism was studied in 862 Swedish men adopted by nonrelatives at an early age. Both the congenital and postnatal backgrounds of the adoptees modify their risk for alcohol abuse. We distinguish two forms of alcoholism that have distinct genetic and environmental causes and differ in their association with criminality, severity of alcohol abuse, and the frequency of expression in biological mothers. Postnatal milieu determines the frequency and severity of expression of the common type of susceptibility in both men and women. In contrast, the less common type is highly heritable in men but is seldom expressed in mothers of affected men.