In a cohort of 60 436 women with a diagnosis of invasive breast carcinoma and known to reside in Sweden in 1960, 321 had a subsequent diagnosis of ovarian carcinoma. Assuming no correlation between the two cancers, one would expect that 191 women would develop ovarian cancer (standardized incidence ratio (SIR) 1.7; 95% confidence interval 1.5-1.9). Women with breast cancer before 40 years of age were at highest risk for developing ovarian cancer (SIR 4.5). Between 40 and 49 years of age, the SIR was 1.9, and at 50 years of age or older, the SIR was 1.3. Most of the excess in ovarian cancer occurred in southern Sweden. The geographic distribution of these cases coincided with the distribution of families with known BRCA1 and BRCA2 gene mutations. These results suggest that genetic factors account for the excess in ovarian cancer that occurs in breast cancer patients and that geographic clustering of patients who have both breast and ovarian cancer may indicate the presence of a BRCA founder mutation.