Thirty-four children (ages 6-12 years) with moderate to borderline mental retardation were studied in a laboratory classroom setting to determine whether children identified as having attention deficit hyperactivity disorder on the basis of Conners Questionnaires differed in classroom behavior. Half of the children scored 15 or greater on both the Parent and Teacher Conners; the remaining children scored 11 or less. All were participants in a Saturday Education Program serving children with mental retardation. Direct observation of the laboratory classroom documented significant differences between groups on measures of on-task behavior and fidgetiness, especially during situations where little direct teacher feedback or supervision was available. Saturday Education Program staff, while blind as to group designation, rated the two groups as differing significantly on all scales of two standardized behavior problem checklists. Checklists by parents and teachers appear to be valid measures of classroom behavior of children with moderate to borderline mental retardation.