Family-genetic studies consistently show that attention deficit hyperactivity disorder (ADHD) aggregates in nuclear families. Little is known about the aggregation of ADHD in second-degree relatives. We examined the prevalence of ADHD in second-degree relatives of 140 ADHD probands and 120 normal controls. Information pertinent to the diagnosis of ADHD in second-degree relatives was collected from the probands' parents. The second-degree relatives of ADHD probands were at increased risk for ADHD compared with the second-degree relatives of normal control probands. These risks were greatest when the second-degree relative was biologically related to an ADHD parent of an ADHD proband. Consistent with the greater prevalence of ADHD among boys compared with girls, grandfathers were at greater risk than grandmothers and uncles were at greater risk than aunts. Our results support the usefulness of ascertaining information from second-degree relatives in studies evaluating the genetic epidemiology of ADHD. If confirmed, such studies may help clarify the mechanism of familial transmission of ADHD.