The aim of the present study was to assess the activity of the hazelnut oil in the treatment of polycystic ovary syndrome in rats. Serum follicle-stimulating hormone, luteinizing hormone, estradiol, progesterone, testosterone, serum lipid parameters, leptin and glucose levels were evaluated. Moreover, antioxidant activity was tested using superoxide dismutase, malondialdehyde, catalase, glutathione peroxidase levels. The phytosterol content of the oil was determined by HPLC. The plasma high density lipoprotein-cholesterol level was found to be significantly high and leptin and glucose concentrations were found to be significantly low in the treatment group. According to the phytochemical analysis, the main components of the oil were detected as alpha-tocopherol, gamma-tocopherol, squalene, beta-sitosterol, campesterol and stigmasterol. Corylus avellana oil was found to be effective in the treatment of polycystic ovary syndrome via regulating gonadotropins, steroids and serum lipid parameters and possesses antioxidant activity. (C) 2015 Sociedade Brasileira de Farmacognosia. Published by Elsevier Editora Ltda. All rights reserved.