An investigation of the genetic epidemiology of breast cancer involving complex segregation analysis of 200 breast cancer pedigrees of Danish extraction is presented. The observed distribution of breast cancer is compatible with transmission of an autosomal-dominant gene with no evidence for residual family resemblance. The gene frequency of the abnormal allele is 0.00756, and the displacement between the homozygous genotype means is 1.695. The gene frequency accounts for a significant proportion of breast cancer in young women, whereas by an advanced age a majority (87%) of affected women are phenocopies. Genetic modeling of other breast cancer families and results of linkage studies are reviewed.