The study examined the 2-year stability and validity of playroom observations of behaviors associated with hyperactivity in 48 boys who were evaluated originally as outpatients in a child psychiatry clinic, and seen again approximately 2 years later at follow-up. At both evaluations, the boys participated in two 15-minute playroom procedures: a relatively unstructured free play period; and a relatively structured, restricted academic period. The observation measures of gross and fine motor activity, sustained attention, and self-vocalization in both periods showed significant stability over the 2-year period, even after the effects of age and IQ were partialled out. Further, these behaviors generally showed significant improvement between the two observations. Thus, consistent with previous research and clinical observations, behaviors typically associated with hyperactivity were found to show significant improvement as the boys got older, although their relative rank order remained stable. The results also indicated that a measure of hyperactivity generated at referral was significantly related to many of the observation measures collected at follow-up. Taken together, these results suggest that the present playroom observation procedure is measuring stable and valid aspects of hyperactive behavior among a sample of clinic-referred boys. (Author/CL)