e13041 Background: Currently, the role of BRCA1/2 in the development of primary and multiple malignant breast tumors is under discussion, along with the effectiveness of targeted therapy for this oncopathology. At the same time, there is a lack of sufficient research dedicated to the treatment of patients with synchronous breast cancer (BC) concurrent with ovarian cancer (OC) and the outcomes of using PARP inhibitors in this patient population. Methods: A retrospective analysis of treatment outcomes based on long-term survival indicators was conducted for 23 patients with synchronous BC, where the second tumor was OC. These patients underwent evaluation and treatment between 2017 and 2022. The average age of the patients was 45.3±5.6 years, with a prevalence of patients in stages IIb-IIIa of BC and IIb-IIIc of OC. Germline mutations in the BRCA1/2 genes were detected in 8 patients, among whom 5 exhibited the triple-negative phenotype of BC. The 1st group included 11 (47.8%) patients without BRCA1/2 mutations, who received neoadjuvant chemotherapy with paclitaxel 175 mg/m2 + carboplatin AUC 6 once every 3 weeks. In the 2nd group, consisting of 12 (52.2%) patients, olaparib 800 mg was added to this chemotherapy regimen. This group also included 8 patients with BRCA1/2 mutations. Results: In the majority of patients in both groups, a significant elevation in CA125 levels in serum was observed, averaging 563 U/mL, with practically normal CA153 levels at 32 U/mL. Identified mutations included 5382insC and 4154delA mutations in the BRCA1 gene, as well as the 6174delT mutation in the BRCA2 gene. Patients underwent three courses of treatment, and four weeks after completing the last course, surgical intervention was performed in the form of radical mastectomy, hysterectomy, bilateral salpingo-oophorectomy, and omentectomy. Following surgical treatment, patients received adjuvant chemotherapy following the same regimens. In the 2nd group, patients continued to receive olaparib for one year, and no signs of unacceptable toxicity were reported. The median overall survival in the first group of patients was 34.2 months (95% CI: 28.6-37.5), while in the second group, it was 36.8 months (95% CI: 31.4-41.1). The addition of bevacizumab significantly increased the median time to progression from 13.8 months (95% CI: 11.8-14.5) to 15.2 months (95% CI: 12.4-15.8), p=0.042. Lethal outcomes primarily occurred due to the progression of ovarian cancer. Conclusions: Thus, the addition of the PARP inhibitor olaparib to a platinum-based chemotherapy regimen in patients with synchronous breast cancer, associated with germline mutations in BRCA1/2 genes and the presence of ovarian cancer as a second tumor, has a positive impact on treatment outcomes. This contributes to the resectability of patients and significantly increases the median time to progression.