A homozygous mutation in the RAD51C gene was recently found to cause Fanconi anemia-like disorder. Furthermore, six heterozygous deleterious RAD51C mutations were detected in German breast and ovarian cancer families. We screened 277 Finnish familial breast or ovarian cancer patients for RAD51C and identified two recurrent deleterious mutations (c.93delG and c.837+1G>A). These mutations were further genotyped in 491 familial breast cancer patients, 409 unselected ovarian cancer patients and two series of unselected breast cancer cases (884 from Helsinki and 686 from Tampere) and population controls (1279 and 807, respectively). The mutation frequency among all breast cancer cases was not different from the controls (4 out of 2239, 0.2% versus population controls 2 out of 2086, 0.1%, P= 0.7). In the Helsinki series, each mutation was found in four cases with personal or family history of ovarian cancer. No mutations were found among cases with familial breast cancer only, four out of the eight carriers did not have family history of breast cancer. The mutations associated with an increased risk of familial breast and ovarian cancer (OR: 13.59, 95% CI 1.89-97.6, P= 0.026 compared with controls), but especially with familial ovarian cancer in the absence of breast cancer (OR: 213, 95% CI 25.6-1769, P= 0.0002) and also with unselected ovarian cancer (OR: 6.31, 95% CI 1.15-34.6, P= 0.033), with a significantly higher mutation rate among the familial cases (two out of eight, 25%) than the unselected ovarian cancer cases (4 out of 409, 1%) (OR: 33.8, 95% CI 5.15-221, P= 0.005). These results suggest RAD51C as the first moderate-to-high risk susceptibility gene for ovarian cancer.