The inheritance of alcohol abuse and other psychopathology was studied in 862 men and 913 women adopted by nonrelatives at an early age in Sweden. Both male and female adoptees had a greater risk of alcohol abuse if their biologic, but not adoptive, parents were alcoholic. We distinguished two types of alcoholism that have distinct genetic and environmental causes and that differ in frequency of alcohol abuse and somatoform disorders in women. The combination of both genetic and environmental risk factors is required for development of alcoholism in the most common type. In contrast, in families with the less common type of susceptibility, alcohol abuse is highly heritable in the men, but the women have multiple somatic complaints without alcohol abuse. The implications of these findings of genetic heterogeneity and gene-environment interaction are discussed in relation to research, prevention, and treatment.