From February 1996 to April 1998, 2967 women received screening for breast cancer in the gynecologic ambulatory practice of the Hokkaido University Hospital. In 116 Japanese women with epithelial ovarian cancer, mutation analysis of BRCA1 exon 11 in genomic DNA was performed by the stop codon (SC) assay and DNA sequence analysis. Clinicopathological factors were also investigated in these patients. The aim of this study was to examine the advantages of performing BRCA1 mutation testing for ovarian cancer patients during breast cancer screening. We achieved a high detection rate (6.0%) of patients with germline mutations in BRCA1. The high frequencies of breast ovarian cancer syndrome, serous adenocarcinoma, high histological grades, advanced FIGO stages, and breast cancer as double cancer were found to be characteristic of ovarian cancer with germline mutations in BRCA1. These characteristics may assist physicians in selecting BRCA1 mutation testing for ovarian cancer patients. The mean age at diagnosis of ovarian cancer was 51.0 and 51.2 years in the groups with and without mutation, respectively, and no difference was found in age at diagnosis. All of the nine living female mutation carriers were offered the options of increased surveillance or prophylactic surgery, and all chose the former. We have performed breast cancer screening and/or ovarian cancer screening every 6 months for these carriers. This may allow another advantage in establishing a relationship of mutual trust with a patient from a series of responsible follow-ups.