This study assessed the relationship between parent and teacher ratings of hyperactivity and developmental indices (chronological age, IQ, and mental age) in two groups of children. Subjects were drawn separately from two psychiatric clinics: (a) a general clinical sample, largely of normal ability, seen for a multitude of psychiatric and behavioral problems, and (b) a developmental clinic sample, comprising children with cognitive delays and seen for ADHD or the absence of ADHD. The results showed consistent negative correlations between chronological age and severity of hyperactivity symptoms; however, these occurred mainly within the developmental clinic sample. Only 4 of 27 comparisons (15%) between mental age and hyperactivity ratings (all confined to the developmentally delayed sample) showed a significant correlation. When chronological age was first partialled out, the correlations between ratings of hyperactivity and mental age ceased to be significant. Findings suggest that chronological age should be taken into consideration when behavior ratings are used to assess cognitively delayed children for ADHD. However, the results do not support guidelines stating that mental age must be used to determine which norms should be applied when such children are evaluated clinically.