Background and Objectives: The objectives of this study were to investigate the effects of cinnamon supple- mentation on glycemic indices, serum lipids and adiponectin in patients with polycystic ovary syndrome (PCOS). Methods and Study Design: This double-blind randomized controlled clinical trial was conducted on 84 over- weight or obese PCOS patients. Subjects in cinnamon (n=42) and placebo (n=42) groups were given 3 cinnamon capsules (each one contained 500 mg cinnamon) or placebo daily for 8 weeks. Fasting blood samples, anthropo- metric measurements and dietary intake data were collected at the baseline and at the end of the trial. Data were analyzed by independent t test, paired t test and analysis of covariance. Results: Cinnamon significantly de- creased serum fasting blood glucose, insulin, homeostatic model assessment for insulin resistance, total choles- terol and low-density lipoprotein cholesterol and weight and increased high-density lipoprotein cholesterol com- pared with placebo (all p<0.05). Serum triglyceride and body mass index significantly decreased in the cinnamon group, in comparison with baseline values (p=0.001 and p=0.002, respectively). No significant changes were seen in serum adiponectin in either group. Conclusions: Short term supplementation of cinnamon had some favoura- ble effects on metabolic risk factors of women with PCOS and may be useful in management of PCOS complica- tions.