Attention-deficit/hyperactivity disorder (AD/HD) is a common behavioral disorder among children, yet there is uncertainty about many aspects of AD/HD, including its precise frequency of occurrence, presence of comorbid conditions, long-term outcomes and the impact of treatment on long-term outcome. The objectives of the studies described in this paper were to address these important questions about AD/HD. The study subjects were from a population-based birth cohort, and included all children born in Rochester, Minnesota, from 1976 to 1982, who remained in the community after the age of 5 y (n = 5718). Complete medical and school records were available for all members of the birth cohort, including detailed information about AD/HD symptoms, psychometric test and questionnaire results, medical and non-medical treatments, and long-term academic and psychosocial outcomes. This information was used to identify 375 subjects from the birth cohort with AD/HD, based on well-defined research criteria. The estimated cumulative incidence of AD/HD at the age of 19y was 7.5% (95% confidence interval 6.5-8.4). Data from these 375 subjects with AD/HD will be used in ongoing studies of this disorder.