This paper reports on results of the EORTC protocol 30892, an open, prospective, randomized study of 310 patients with previously untreated metastatic prostate cancer with favourable prognostic factors who were treated by either flutamide (FLU) or cyproterone acetate (CPA) monotherapy. The final analysis with regard to the main end points, time to progression and survival are still pending. Final results related to the evaluation of sexual functioning prior to and under treatment are reported here. Of 310 randomized patients 294 were eligible for evaluation within this side study. The median age was 71 years (range 48-85). Potential risk factors related to age, general health and prostate cancer were evaluated. For evaluation of sexual functions a five-item questionnaire was used which was administered by the investigator. The protocol allowed time dependent observations at 3-monthly follow-up visits. Sexual functioning was dependent on age but not on prostate cancer-related parameters. Sexual functions at entry were similar within the two treatment groups, spontaneous (nightly) erections and sexual activity were seen in 43-51% and 29-35% of cases. Under treatment, sexual functions under FLU and CPA declined slowly with median times of 12.9 and 5.8 months versus 13.7 and 8.9 months respectively for spontaneous erections and sexual activity. Eventually, with an average observation time in excess of 2 years, loss of spontaneous erections and of sexual activity occurred in 80% versus 92% and in 78% versus 88% of men under FLU versus CPA treatment respectively. None of these differences reached statistical significance. Maintenance of potency under treatment with FLU as reported in the literature is not confirmed in this study. However, loss of sexual functions under monotherapy with both antiandrogens is slow and 10-20% of men retain sexual activity after 2-6 years of treatment. This observation can be exploited in new treatment schemes and is likely to lead to improved quality of life. The advantage of FLU in time and total preservation of sexual functions is statistically not significant and must be balanced against the side effects of FLU and other pure antiandrogens, which may exceed those of CPA especially with respect to gynaecomastia. Hepatic toxicity may limit the long-term use of both drugs.