Introduction: Attention deficit hyperactivity disorder (ADHD) is the most prevalent neurodevelopmental disorder among children and adolescents. Given the importance of this disorder, the aim of this study was to investigate the prevalence of ADHD and the relevant factors among the elementary school students in Ardabil, Iran. Methods: Using the stratified random cluster sampling method, this cross-sectional descriptive study was conducted on 2826 schoolchildren aged 6-11 in Ardabil, in the year 2011-2012. The necessary information was collected in the first step employing the ADHD questionnaire based on 18 Diagnostic and Statistical Manual of Mental Disorders-4 th edition (DSM-IV) criteria filled out by parents and teachers. Then the students were interviewed by psychiatrists in the second step. SPSS was used to analyze the research findings. Results: The research results indicated that the lifetime prevalence of ADHD was 9.8%. The most prevalent type of ADHD was the hyperactive-impulsive (6.8%). The results also indicated that the prevalence of ADHD was higher in boys than girls. One-third of affected children were premature at birth and 49.2% had attended kindergartens. The ADHD group was significantly different from the normal students in terms of the average discipline score and the overall grade at school. Conclusion: There is a relatively high prevalence of ADHD in elementary school students; therefore, it is strongly recommended to identify and treat it appropriately in the years prior to school.