This study examined patterns of comorbid/associated diagnoses and associated problems in a population sample of children with and without DSM-III-R attention-deficit hyperactivity disorder (ADHD). Half (N = 409) of a mainstream school population of Swedish 7-year-olds were clinically examined, and parents and teachers were interviewed and completed questionnaires. The children were followed up 2–4 years later. Eighty-seven per cent of children meeting full criteria for ADHD (N = 15) had one or more—and 67% at least two—comorbid diagnoses. The most common comorbidities were oppositional defiant disorder and developmental coordination disorder. Children with subthreshold ADHD (N = 42) also had very high rates of comorbid diagnoses (71% and 36%), whereas those without ADHD (N = 352) had much lower rates (17% and 3%). The rate of associated school adjustment, learning, and behaviour problems at follow-up was very high in the ADHD groups. We concluded that pure ADHD is rare even in a general population sample. Thus, studies reporting on ADHD cases without comorbidity probably refer to highly atypical samples. By and large, such studies cannot inform rational clinical decisions.