According to previous data, the addition of risperidone in obsessive-compulsive patients refractory to serotonin reuptake inhibitors (SRIs) is shown to be a safe and effective treatment strategy. The aims of our study were to evaluate the efficacy of risperidone addition, in comparison to placebo, in fluvoxamine-refractory obsessive-compulsive patients and to investigate whether risperidone could boost the efficacy of fluvoxamine in fluvoxamine-responder patients. Subjects were 45 obsessive-compulsive inpatients, consecutively recruited at the Department of Neurosciences at the San Raffaele Hospital, Milan. Thirty-nine patients completed the study. All patients received 12 weeks of a standardized open-label fluvoxamine monotherapy and then continued for 6 weeks with placebo or risperidone in a double-blind design. Results showed a significant effect of risperidone addition, at the end of the double-blind phase (18th week), only for fluvoxamine-refractory patients. Five patients on risperidone (50%) and two (20%) on placebo became responders, with a Yale-Brown Obsessive-Compulsive Scale (Y-BOCS) decrease > or =35%. Risperidone was generally well tolerated, except for a mild transient sedation and a mild increase in appetite. This preliminary study suggests that even very low (0.5 mg) risperidone doses are effective in OC patients who were nonresponders to a standardized treatment with fluvoxamine.