This study investigated the relationship of the behavioral observation system (BOS; LePage & Mogge, 2001) and clinician ratings of psychosis and mania. Fifty interviews with psychiatric inpatients using the Scale for the Assessment of Positive Symptoms, the Scale for the Assessment of Negative Symptoms, and the Young Mania Rating scale were completed. Paraprofessionals completed a BOS on the patient during the same day of the clinician's interview. The results of the study demonstrate the convergent validity of the BOS scales and support the use of the BOS by paraprofessionals to assess behaviors associated with psychosis and mania. © 2009 Wiley Periodicals, Inc. J Clin Psychol 66: 333–338, 2010.