Purpose of Investigation: To analyze a relationship between histological grade of ductal breast cancer (BC) and intratumor concentration of serotonin in women with polycystic ovarian syndrome (PCOS), and to study the long-term effects of innovative neurohormonal therapy in this group. Materials and Methods: The study included 65 women (28 premenopausal and 37 postmenopausal, age 43-69 years) with PCOS and BC. Aside from mastectomy, chemotherapy and menopausal status-specific hormonal treatment, all subjects received metformin, quinagolide, melatonin, and methysergide maleate. Serum concentrations of sex hormones were determined post-mastectomy and after five and ten years. Results: Intratumoral concentrations of serotonin in grade II and III BC were higher than in grade I tumors. After ten years, all subjects had normal concentrations of estrogens, prolactin, and serotonin, and showed no signs of BC recurrence. Conclusion: Intratumor level of serotonin may serve as a marker of BC aggressiveness. Neurohormonal therapy may prevent progression of BC.