Objective Clarifying the characteristics of attention-deficit/ hyperactivity disorder (ADHD) symptoms in childhood is important for the prevention and management of this disorder. The purpose of this study was to determine the prevalence of ADHD symptoms in Japanese preschool children based on evaluations performed by parents or teachers. Methods A questionnaire survey was performed to evaluate the estimated prevalence of ADHD symptoms in preschool children in Niigata City, Japan. The first survey, conducted in 2003, involved an evaluation of ADHD symptoms by their school teachers. The second survey, conducted in 2006, involved an evaluation of the symptoms by parents. The teacher survey included 9,956 children, and the parent survey included 7,566 children. Parents and teachers assessed ADHD symptoms in children using a 14-item questionnaire based on DSM-III-R. Children with a score of 8 or higher were classified as having ADHD symptoms. Results The overall prevalence of ADHD symptoms was 2,349/7,566 (31.1%) in the parent survey and 431/9,956 (4.3%) in the teacher survey, with a prevalence ratio of 7.2 (95% CI: 6.5-7.9). Likelihood ratio test indicated that variables significantly associated with the presence of ADHD symptoms were gender, age, school type, interaction between gender and observer, and interaction between school type and observer (each with P \ 0.0001). ConclusionsThe large difference between the estimated prevalence of ADHD symptoms in Japanese preschool children from teacher and parent surveys suggests that compared to teachers, parents consider their children's symptoms much more serious. Thus, parental evaluation of ADHD symptoms using DSM criteria may be inappropriate for ADHD screening.