Treatment of aggression in schizophrenic patients is a major challenge. We sought to examine the efficacy of augmentation of antipsychotic treatment with pindolol in the amelioration of aggression. Thirty male inpatients meeting DSM-IV criteria for schizophrenia, aged 20-65 years involved in four or more aggressive incidents in the two previous months, were enrolled in a double-blind crossover study. Aggression was evaluated per incident, with the Overt Aggression Scale (OAS). Positive and Negative Syndrome Scale (PANSS) was administered at baseline, crossover and at endpoint. Patients received either pindolol or placebo augmentation 5 mg x three times a day until crossover, then switched. No significant differences were found in the PANSS scores between the placebo and pindolol treatments. OAS scores were significantly reduced for number of aggressive incidents towards objects and other persons during pindolol treatment (0.59 versus 1.46, F = 6.09, P < 0.02; 1.96 versus 3.23, F = 4.17, P < 0.05, respectively). Similar results were obtained for severity of incidents (0.89 versus 3.58, F = 19.42, P < 0.0001; 2.89 versus 6.85, F = 10.11, P < 0.004, respectively). Pindolol, with its dual beta and 5-HT1A blocking effect ameliorated both number and severity of aggressive acts. Influence on severity may be associated with a 5-HT1A antagonistic effect.