Gynecomastia (breast enlargement) is a side effect of neuroleptic antipsychotic drugs, related to prolactin elevation caused by dopamine D2 receptor blockade (Richelson, 1996). The atypical antipsychotic risperidone is less likely to cause gynecomastia at low doses (Casey, 1996). It can cause a dose-dependent increase in serum prolactin concentration (Peuskens, 1995), by blocking dopamine D2 receptors (Richelson, 1996). I would like to describe a patient who did not have gynecomastia with risperidone at a dose of 3 mg/day, but had it when risperidone, at a dose of 0.5 mg/day, was combined with fluoxetine. A MEDLINE search failed to find any reports about such an interaction.