Serum testosterone levels decline significantly with aging and this has been associated with reduced sexual function. We have conducted a double-blind, randomized, placebo-controlled trial to investigate the effect of testosterone supplementation on sexual function in 237 elderly men with a testosterone level <13.7 nmol l(-1). Participants were randomly assigned to receive oral testosterone undecanoate or a placebo for 6 months. A total of 207 men completed the study. After treatment, there were no differences in scores on sexual function between the groups. Subanalysis showed that although a baseline testosterone level in the lowest tertile was associated with significantly lower scores for sexual fantasies, desire of sexual contact and frequency of sexual contact, supplementation of testosterone did not result in improvement on any of these items in this group. In conclusion, the findings do not support the view that testosterone undecanoate supplementation for 6 months to elderly men with low-normal testosterone concentrations favorably affects sexual function.