The objective of this study was to compare sexual functioning in patients treated with olanzapine or risperidone. This open-label trial included 46 patients randomized to olanzapine (5-15 mg/d) or risperidone (1-6 mg/d) for 6 weeks. We used sexual dysfunction was assessed by a semistructured interview based on the items of the UKU side effect rating scale. Three olanzapine-treated patients (12.0%), compared with 11 risperidone-treated patients (52.4%), reported sexual dysfunctions (p = .008) in the semistructured interview. Only 4 patients (8.7%) spontaneously reported sexual dysfunction. The mean dose was 9.4 mg/d for olanzapine and 3.4 mg/d for risperidone. The mean (+/-SD) prolactin levels (ng/mL) in olanzapine-and risperidone-treated patients were 25.1 (+/- 23.5) and 43.5 (+/- 26.1), respectively. Less sexual dysfunction occurred in the group treated with olanzapine compared with the risperidone group. Direct questioning about sexual functioning is necessary to avoid underestimating the frequency of sexual side effects in patients with schizophrenia and related psychotic disorders.