This study examined the initial reliability and validity of the Behavioral Observation System (BOS). The BOS, which consists of 34 items rating inpatient behaviors and is completed by psychiatric aids, was developed to assess the domains of Psychosis, Mania, Depression, and Behavioral Dyscontrol. Three studies are described. Results of the reliability study suggest adequate intraclass correlations and coefficient alphas. Analyses of the interscale correlations demonstrated appropriate initial discrimination between the scales. A comparison of diagnoses with ratings yielded expected convergent validity. The overall results suggest the BOS is likely to provide reliable and useful information to treatment teams. Directions of future studies also are discussed.