Background and objective: Polycystic ovarian syndrome (PCOS) is the most common endocrine disorder among reproductive-age women. There is very little information about the prevalence of PCOS in Iran. With regard to the symptoms of PCOS which begin after menarche and regarding to its side effects on women's health, we aimed to determine the prevalence of PCOS in adolescents in Zanjan, Iran. Materials and Methods: In this descriptive community based study, 1882, 14-18 year old adolescents were randomly selected from Zanjan schools. The presence of PCOS was determined by the presence of olygomenorea, hirsutism, acne and androgenic alopecia. For correlation between PCOS and obesity, BMI and central obesity was evaluated. Statistical analysis was performed using K2 test. Results: PCOS was present in 54(2.9%), hirsutism was present in 161 (8.6%), acne was present in 220 (11.7%), androgenic alopsia was present in 130(6.9%) and menstrual irregularity was present in 281 (16.9%) of the cases. The prevalence of central obesity and over weight did not differ among the studied groups. Conclusion: The prevalence of PCOS in our study was similar to the results reported from other societies. With regard to PCOS side effects, we suggest that the diagnosis and treatment of PCOS is better to be started from adolescence.