OBJECTIVE:The present study examined cognitive functioning in a population sample of adolescents with and without attention-deficit/hyperactivity disorder (ADHD) from the Northern Finland Birth Cohort 1986. METHOD:The sample consisted of 457 adolescents ages 16 to 18 who were assessed using a battery of cognitive tasks. Performance according to diagnostic group (control, behavior disorder, and ADHD) and sex was compared. Then, the effect of executive function deficit (EFD) was assessed by diagnostic group status on behavioral and cognitive measures. RESULTS:When compared to non-ADHD groups, adolescents with ADHD exhibited deficits on almost all of the cognitive measures. The behavior disorder group obtained scores that were generally intermediate between the ADHD and control groups, but exhibited deficits in intelligence and executive function similar to the ADHD group. Approximately half the ADHD sample had EFD; however, the type and presence of EFDs were not differentially related to cognitive performance as a function of diagnosis. CONCLUSIONS:These findings indicate that EFDs are more frequent in ADHD than control or behavior disorder groups. EFDs are a general risk factor for poor cognitive functioning across multiple domains, irrespective of diagnostic status.