Positive psychotic symptoms, such as threat/"control-override" delusions or command hallucinations, have been related to aggression in patients with schizophrenia. However, retrospective data collection has hampered evaluation of the direct influence of psychopathology on aggressive behavior. In this study, we monitored aggressive behavior and psychopathology prospectively and in close temporal proximity in 157 treatment-resistant inpatients diagnosed with chronic schizophrenia or schizoaffective disorder participating in a 14-week double-blind clinical trial. Aggressive behavior was rated with the overt aggression scale (OAS). Psychopathology was assessed using the positive and negative syndrome scale (PANSS). At baseline, subjects who would be aggressive during the study had higher scores on only two PANSS items: hostility and poor impulse control. During the study PANSS positive subscale scores were significantly higher in aggressive subjects. Total PANSS scores were higher within 3 days of an aggressive incident, as were positive and general psychopathology subscale scores. However, in a smaller subsample for whom PANSS ratings were available within 3 days before aggressive incidents, only scores on the PANSS positive subscale were significantly higher. These findings in chronic, treatment resistant inpatients support the view that positive symptoms may lead to aggression.