Mutations of BRCA1 and BRCA2 genes account for the majority of hereditary breast and ovarian cancers. So far, risk-reducing salpingo-oophorectomy has been the most effective strategy for gynecological cancer prevention in susceptibility gene mutation carriers. It does not prevent, however, from the occurrence of primary peritoneal cancer. We present two clinical cases of patients with the BRCA1 gene mutation. Both patients had a family history of cancer and both were presenting with metachronic malignances. The first patient, whose mother suffered from breast and ovarian cancer, was diagnosed with left breast cancer in 2004. The patient was 44 years old at diagnosis. Genetic testing revealed the BRCA1 gene mutation. A breast conserving therapy (BCT) was conducted, followed by chemotherapy, radiotherapy and immunotherapy with trastuzumab due to HER2 overexpression. Due to BRCA1 mutation, in November 2005, prophylactic hysterectomy with appendages was performed. Histological examination revealed bilateral ovarian cancer (adenocarcinoma G3) with metastasis to the paraaortal lymph node. The patient received six cycles of chemotherapy: paclitaxel and carboplatin. Ovarian cancer relapsed 3 years later. After that the patient received 5 lines of chemotherapy and finally died due to disease progression in September 2011. The second patient, a 49-year-old woman, was diagnosed with breast cancer in July 2003 and subsequently treated with neoadjuvant chemotherapy, breast conserving surgery and radiotherapy. Genetic testing was also performed and revealed the BRCA1 gene mutation. A year earlier the patient had undergone hysterectomy with appendages due to uterine myomas. Three of her five sisters suffered from breast and ovarian cancer. The patient’s father died of colorectal cancer. The patient remained under surveillance. Because of the increasing level of Ca-125 (since October 2004), PET-CT was performed and revealed a tumor lesion of the peritoneum. Histological examination from the biopsy confirmed primary peritoneal cancer (papillary serous adenocarcinoma – primary peritoneal carcinoma). Reexamination of the tissues from hysterectomy with appendages was also performed and revealed an adenocarcinoma in the right ovary. Pathologic examination excluded metastasis of a breast cancer. Pathomorphology of the ovarian lesion was also different than in the lesions of the peritoneum. Thus, three different tumor types (breast, ovarian and peritoneal cancer) coexisted independently. The patient received chemotherapy: paclitaxel and cisplatin. Later on, due to disease progression she was treated with five consecutive chemotherapy regimens and hormonal therapy. The patient died in January 2008. These case illustrate that genetic diagnosis may be critical for the overall treatment plan.