The objective of the study was to compare polysomnographic patterns in two groups of preadolescent children with attention deficit hyperactivity disorder (ADHD) (a sleep clinic referral sample and a community sample) with control children. A prospective and observational study in the sleep clinic and the community was undertaken. Forty-seven ADHD children referred to the sleep clinic (ADHDcl), 53 ADHD children from a community survey (ADHDcom), and 49 control children underwent overnight polysomnography. Significant differences between the groups were observed for rapid-eye-movement (REM) sleep latency and percentage, and periodic limb movement index with associated arousals (PLMa). REM sleep latency was shorter in controls than ADHDcl and ADHDcom (p < 0.01) and REM% was highest in controls and lowest in ADHDcl (p < 0.001). PLMa was higher in ADHDcl than the other groups (p < 0.001), but there were no differences in PLMa between ADHDcom and controls. ADHD children display significant alterations in their sleep patterns, and ADHDcl are more likely to have an elevated PLMa than ADHDcom. In addition, REM sleep is affected by ADHD. We postulate that ADHDcl may represent a subset of children with ADHD at high risk for hyperactivity during sleep.