This study was aimed at determining whether the familial clustering often observed in breast cancer is associated with genetic factors. We compared familial and sporadic breast cancer cases with respect to ABO blood group distribution and the risk of bilateral disease, using the data from the Icelandic Cancer Registry which contains genealogical information for about 30% of the breast cancer cases diagnosed in Iceland since 1911. Cases were classified as familial if at least one first-degree relative had breast cancer. Using this criterion, we identified 184 familial cases and 572 sporadic cases. The familial cases had a 2-fold higher prevalence of blood group B than did the sporadic cases, and the frequency of this blood group in non-affected relatives of cases was significantly reduced. Familial cases were about 2.7 times more likely to suffer from bilateral breast cancer than were the sporadic patients. These results support the presence of a genetic factor in the etiology of familial breast cancer.