Aggressive and impulsive behavior is frequently observed in schizophrenic patients. Previous studies suggest that impulsive aggression may be the most common behavioral correlate of central serotonergic system dysfunction. This study was aimed to determine if clozapine, an atypical antipsychotic agent with potent serotonergic antagonistic properties, can reduce impulsiveness and aggression in neuroleptic-resistant chronic schizophrenic patients. Fourteen neuroleptic-resistant chronic schizo- phrenic patients were treated with clozapine and prospectively evaluated for aggressiveness and impulsiveness for 18 weeks. Clozapine treatment induced a marked decrease in impulsiveness (32% on the Impulsivity Scale; p <0.0001) and aggressiveness (98% on the Overt Aggression Scale; p <0.0001). We conclude that clozapine treatment may be effective in reducing psychotic symptoms as well as in controlling aggressive and impulsive behavior in neuroleptic-resistant chronic schizophrenic patients.