Background. There is currently a debate about the diagnostic criteria for polycystic ovary syndrome (PCOS) in adolescent girls. The role of vitamin D in the pathogenesis of PCOS remains unclear. The purpose: to study the serum level of 25(OH)D in adolescent girls and to analyze its connection with PCOS. Materials and methods. Twenty-three patients aged 14–18 years with PCOS were examined on the basis of the Chernivtsi Regional Clinical Hospital. The control group consisted of 25 girls of a similar age without disorders of gynecological status. Body mass index, degree of hirsutism, hormonal, lipid profile, HOMA index, calcium concentration were determined according to generally accepted methods. Levels of serum 25(OH)D were measured using the electrochemiluminescence method. Sonography of the ovaries was performed. Statistical data were processed using the Microsoft Excel 2020 program on a personal computer and the package of application programs Statistica for Windows v. 7.0 (StatSoft Inc.). Ethical principles are observed in the work, taking into account the main provisions of the ІCH GCР and the Declaration of Helsinki. Results. The premorbid background of the examined adolescent girls included allergic reactions, infectious, respiratory, digestive, and urinary diseases. The level of 25(OH)D was inversely proportional to body mass index (r = –0.23, p < 0.05), hirsutism (r = –0.22, p < 0.05), HOMA-IR (r = –0.198, p < 0.05). Deficiency of 25(OH)D in adolescent girls with excess body weight was associated with dyslipidemia (r = –0.33, p < 0.01). Indicators of the serum level of calcium varied within the reference values. The odds ratios for the development of PCOS in adolescent girls were: 25(OH)D [2.35, 95% CI: 1.84–5.06, p < 0.05]; excess body weight [2.09, 95% CI: 1.94–4.56, p < 0.05], clinical hyperandrogenism (hirsutism) [2.98, 95% CI: 2.12–4.19, р < 0.01]; HOMA-IR [2.30, 95% CI: 1.02–5.15, р = 0.04]; high-density lipoproteins [2.56, 95% CI: 1.92–5.87]. Conclusions. PCOS occurs on the background of 25(OH)D insufficiency or deficiency. 25(OH)D deficiency, excess body weight, clinical hyperandrogenism, HOMA-IR, and high-density lipoproteins increase the risk of PCOS in adolescent girls.