Attention deficits have been demonstrated in children with attention deficit hyperactivity disorder (ADHD) in the general population, but little is known about attention deficits in children with ADHD who also have mental retardation. In this investigation, children with ADHD and mental retardation were compared to those with mental retardation without ADHD on tasks assessing sustained and selective attention. Although children with ADHD and mental retardation made fewer correct target detections and more commissions on a vigilance task, no compelling evidence emerged for sustained attention deficits. However, evidence did emerge that was suggestive of selective attention deficits in these children. Results also suggest that girls with mental retardation may be at a higher risk for ADHD than are girls in the general population.