Although research on childhood OCD has increased dramatically over the past 15 years, OCD is referred to as a “hidden epidemic”, primarily because the disorder is frequently unrecognized and is, therefore, under diagnosed. Recent epidemiological studies of child and adolescent OCD suggest the prevalence rate is approximately 1 in 200. This study was under taken to determine the prevalence of OCD among Iranian children and adolescents aged 7 to 17 and to study their association with factors such as sex, birth order, education, separation from mother more than 6 weeks, parents’ relationship, birth complications, socioeconomic status of the family, birth complications, the familial, educational or environmental stress, the probability of the father or the mother being affected by the OCD, and common obsessive and compulsive symptoms. A cross-sectional nationwide epidemiological study of the Iranian population aged 7-17 was designed to estimate the prevalence of OCD and their association with the above mentioned factors. At the first, 28 cities were selected and among each city six schools (three for boys and three for girls) in the level of elementary, guidance and high school) were selected through a randomized systematic and cluster sampling method. Diagnostic and Statistical Manual of Mental Disorders-IV (DSM-IV) criteria were used in diagnosis of OCD. Twenty clinical psychologists interviewed the selected subjects face to face at their schools. Among 7326 students, the prevalence of OCD is.9% (.8% and 1.1% in boys and girls populations).48.31% of the survey samples were boys, 51.69% girls, all of the above-mentioned factors were examined.