Aggressive behavior is difficult to capture with most available rating instruments because it occurs with low frequency. The Overt Aggression Scale (OAS) was designed to capture all incidents on a 24-hour basis. To assess its usefulness, the OAS was piloted on 16 hospitalized, severely aggressive children with conduct disorder. OAS ratings capture episodes of aggressive behavior, measure trends in aggressive behavior over time, and appear to reflect change associated with pharmacotherapy in hospitalized children. This study suggests that the use of the OAS on a children's psychiatric inpatient unit is appropriate and feasible for clinical as well as research purposes.