OBJECTIVE The aim of the present study was to investigate the epidemiology of psychiatric disorders in children and adolescents in five provinces of Iran: Tehran, Shiraz, Isfahan, Tabriz and Mashhad. METHODS In the present study, we selected 9,636 children and adolescents aged 6-18 years through multistage cluster random sampling method from Tehran, Shiraz, Isfahan, Tabriz and Mashhad. We instructed the clinical psychologists to complete the Strengths and Difficulties Questionnaire (SDQ) for the participants, andthose who received a high score on SDQ, completed the Persian version of Kiddie-SADS-Present and Lifetime Version (K-SADS-PL). We used descriptive analysis and 95% confidence interval to investigate the relationship between scores of the K-SADS questionnaire and demographic factors. We used one-way ANOVA to test the significant differences among the disorders according to sex, age and province of residence. RESULTS Based on the results, oppositional defiant disorder (ODD) (4.45%) had the highest prevalence of psychiatric disorders in the five provinces and substance abuse and alcohol abuse (0%) had the lowest prevalence. In addition, attention deficit hyperactivity disorder (ADHD) had the most prevalence in boys (5.03%) and ODD had the most prevalence in girls (4.05%). Among the three age groups, 6 to 9 year olds had the highest rates of ADHD (5.69%); 10 to 14 and 15 to 18 year olds had the highest rates of ODD (4.32% and 4.37% respectively). Among the five provinces, Tehran and Mashhad allocated the highest rates of ODD; Isfahan and Shiraz had the highest rates of ADHD; and Tabriz had the highest rates of social phobia. CONCLUSIONS The current study revealed that the overall frequency of psychiatric disorders based on Kiddie-SADS-Present and Lifetime Version (K-SADS-PL) was higher than a similar study. Moreover, in this study, among the five provinces, Tehran and Mashhad allocated the highest rates of ODD; Isfahan and Shiraz had the highest rates of ADHD; and Tabriz had the highest rates of social phobia. Therefore, these percentage of psychiatric disorders in Iran lead us toward a greater use of consultation and mental health services.