Objective The aim of this study was to determine the prevalence of attention-deficit/hyperactivity disorder (ADHD) and oppositional defiant disorder (ODD) in school age children in Sivas province centre and to examine the distribution of symptoms according to subtypes. Method The study population was made up of 1425 children between the ages of 6 and 15 from eight primary schools selected by random sampling method in Sivas province centre. All students were given a questionnaire prepared according to DSM-IV criteria that was completed by their parents or teachers. This form includes 18 symptoms of ADHD and eight symptoms of ODD. Student's t-test and variance analysis were used for statistical evaluation. Results A prevalence of 8.1% was found for ADHD in our study. According to subtypes, 32.2% of these were "inattentive", 40% were "hyperactive" and 27.8% were combined type. A prevalence of 11.5% was found for ODD. The boy/girl ratio was >1 for both ADHD and ODD, and 52.2% of those with ADHD were established as having ODD at the same time. Conclusions The prevalence of ADHD and ODD in primary school children was not considered to be small. Studies concerning the distribution of symptoms in different societies, cultures, and viewpoints and identification of children with this type of problem are beneficial for early diagnosis and prophylactic treatment.