Background. Polycystic ovary syndrome (PCOS) is one of the most common endocrine disorders in women of reproductive age. Recently, the role of melatonin in the pathogenesis of this syndrome became widely discussed among the scientists, because there is an evidence of its impact on the reproductive function and maturation of oocytes. Aim. To study a informativeness of melatonin determination and its relationship with sleep disorders in PCOS. Materials and methods. The study involved 120 women aged 17–35 years: 60 patients with PCOS and 60 women without this disorder as controls. The level of melatonin in the blood, saliva and its metabolite in urine – 6 sulfatoximelatonin were analyzed. To identify sleep disorders survey was conducted using a questionnaire scoring subjective sleep characteristics. Results. Sleep disorders based on subjective scoring profiles sleep characteristics were identified in PCOS group (up to 70% of patients) regardless of BMI. The level of 6-sulfatoximelatonin in urine, nocturnal melatonin levels in saliva (at 3:00 AM) and melatonin in the blood were significantly higher in patients with PCOS compared with the control group regardless of BMI. The level of melatonin in follicular fluid was lower in patients with PCOS. There was a significant correlation of melatonin levels in the blood and the degree of sleep disorders according to the questionnaire scoring subjective sleep characteristics, the level of melatonin in saliva at 3:00 AM and a 6-sulfatoximelatonin in daily urine (p = 0.046). Conclusions. PCOS is polyetiology disease, and an important role in the formation and progression in which plays melatonin. Correlation of levels of this hormone in different body fluids suggests its systemic action and direct involvement in the regulation of reproductive function.