A cancer-related family history was ascertained in 600 women with breast cancer. Prevalence data in the families were compared to the data of the population-based Basel cancer registry. Breast cancer occurrence was increased in first degree female relatives (relative risk: 1.7; 95% confidence interval: 1.4-1.9; p less than 0.0001). Breast cancer risk in relatives decreased with increasing age of the probands. Women at elevated breast cancer risk can be identified by obtaining a family history of breast cancer patients.