Polycystic ovary syndrome is the most common endocrine disorder in women of reproductive age. It has many clinical characteristics and symptoms, and they all vary depending on the patient’s age. Major clinical features of this syndrome include oligo- and/or anovulation, clinical and/or biochemical signs of hyperandrogenism and polycystic ovaries on ultrasound. Disturbed gonadotropine secretion is characterized with LH/FSH inversion and increased insuline secretion with consequent hyperinsulinemia and insulin resistance that are frequently associated with this syndrome. Since it frequently runs in familes it is also considered that genetic factors play a major role in the development of this syndrome. According to these most common symptoms and characteristics Rotterdam criteria (ESHRE/ASRM criteria) have been most widely used for diagnosis of polycystic ovary syndrome. Obesity is a condition that often tracks polycystic ovary syndrome and is found in 38-88% of women with PCOS. The mechanisms by which obesity affects the expression of PCOS symptoms are yet not well defined and they are very complex, but there are a lot of evidences that fat tissue has a very important role in development of PCOS. Elevated body fat tissue in patients with PCOS, especially in those with BMI over 30 kg/m² has a negative effect on all the symptoms of this syndrome. In most of the cases it markly disturbes the insulin resistance, which consequently has a negative impact on all the other PCOS symptoms. Obesity has a negative effect on women’s reproductive health, but also on her health in general, because together with PCOS, obesity often causes metabolic syndrome. To confirm the negative effect of obesity on PCOS it is important to say that weight loss in those patients leads to improvement of clinical symptoms and better regulation of menstrual cycle.