In this study, the prevalence of attention-deficit hyperactivity disorder (ADHD) in kindergarten children in northeast Iran was investigated, and the criterion validity of Conners' parent-teacher questionnaire was evaluated through the use of clinical interviews. This study was a cross-sectional descriptive research project with children in kindergartens that was supported by the Behzisti Organization, which included 155 kindergartens and 4143 children. To study the prevalence of ADHD, cluster random sampling was used; 1083 children were chosen as a reference, and random sampling was used to determine the criterion validity. Twenty-two children scoring below a specified cut-off score and 32 children scoring above the cut-off score were interviewed. In the first stage, ADHD was assessed using Conners' parent-teacher questionnaire, and in the second stage, it was assessed with a clinical interview based on DSM-IV-TR criteria. Finally, the prevalence rate was evaluated with descriptive parameters, and the criterion validity was assessed using the tetracoric correlation coefficient. The prevalence of ADHD was estimated at 12.3% [plus or minus] 2.12%. Moreover, test sensitivity and specificity were evaluated at 90.3% and 81.2%, respectively. Therefore, the criterion validity with [alpha] = 0.05 is 90.3%. The prevalence of ADHD among children was high, suggesting a need to screen all children for ADHD before school age. The Conners' questionnaire proved to be a good test for the primary screening of ADHD among kindergarten children. (Contains 2 tables and 1 figure.)