The genetic epidemiology of ovarian cancer has been investigated by complex segregation analysis of 462 pedigrees ascertained through a normal consultant. The observed pattern of ovarian cancer is compatible with an autosomal dominant gene. The gene frequency of the abnormal allele is 0.0015-0.0026 with a lifetime penetrance of 0.74-0.79. The gene frequency accounts for a significant proportion of ovarian cancer in young women. By age 70 the majority of affected women are phenocopies. The results from this analysis should enable the risks of ovarian cancer to be more accurately estimated than by empiric methods for relatives of affected women, and can maximize the usefulness of screening programmes and future linkage studies.