The evaluation of a patient's potential for aggressive behavior is an important component of care on psychiatric units. In this study, the aggressive behavior of 111 new psychiatric inpatients in Taiwan was recorded by nursing staff using the Overt Aggression Scale (OAS) during the patients' hospitalization. Psychiatrists made diagnoses in accordance with DSM-IV criteria and collected information about psychosocial risk factors for aggression. On average, aggressive patients were hospitalized longer than nonaggressive patients. After duration of hospitalization was controlled for, it was found that patients with an earlier onset of illness were more likely to be aggressive than other patients.