Background and Objective: Polycystic ovary syndrome (PCOS) is the most common endocrine disorder affecting 10–15 % of women in reproductive age. The objective was to evaluate the effect of irisin in the etiology of obese patient with PCOS. Study Design and Methods: The study design its a case control study. This study involved fifty obese ladies (50) with confirmed diagnosis of PCOS. The second group served as control group and included forty apparently normal (non-polycystic ovary) obese ladies (40). The following main parameters were measured: Fasting serum levels of total Cholesterol, triglyceride, high density lipoprotein-cholesterol, low density lipoprotein– cholesterol, glycohaemoglobin (HbA1c%), Insulin , Testosterone, FSH, serum LH and Irisin. Results: The median serum Irisin was significantly higher among PCOS cases (119.1 ng/ml) compared to healthy controls (74.7 ng/ml). The median serum FSH was significantly higher among PCOS cases (6.52 ng/ml ) compared to controls (4.72 ng/ml). Serum LH was obviously higher among PCOS cases (6.88 ng/ml) compared to controls (6.08 ng/ml). The mean pulse rate, blood WBC count and serum VLDL were significantly higher (71.6/min, 8.3 mg/dl and 22.5 mg/dl respectively) in cases with PCOS compared to controls (67.5/min, 7.2 mg/dl and 18.5 mg/dl respectively). Conversely, the serum HDL was significantly lower among PCOS cases (43.8 mg/dl) compared to controls (48.4 mg/dl), serum testosterone showed statistically non-significant differences. Conclusion: Data of the present study showed that serum Irisin was significantly higher among PCOS cases. These findings suggest important role of irisin as a biomarker for PCOS and an important factor in the development of PCOS.