BACKGROUND: Decreased libido is one of several changes in sexual function that are often experienced by female cancer patients. Transdermal testosterone therapy has been associated with increased libido among estrogen-replete women who report low libido. METHODS: In a phase III randomized, placebo-controlled crossover clinical trial, we evaluated whether transdermal testosterone would increase sexual desire in female cancer survivors. Postmenopausal women with a history of cancer and no current evidence of disease were eligible if they reported a decrease in sexual desire and had a sexual partner. Eligible women were randomly assigned to receive 2% testosterone in Vanicream for a testosterone dose of 10 mg daily or placebo Vanicream for 4 weeks and were then crossed over to the opposite treatment for an additional 4 weeks. The primary endpoint was sexual desire or libido, as measured using the desire subscales of the Changes in Sexual Functioning Questionnaire, as assessed at baseline and at the end of 4 and 8 weeks of treatment. Serum levels of bioavailable testosterone were measured at the same times. All statistical tests were two-sided. RESULTS: We enrolled 150 women. Women who were on active testosterone cream had higher serum levels of bioavailable testosterone than women on placebo (mean change from baseline, testosterone versus placebo, week 4, 11.57% versus 0%, difference = 11.57%, 95% confidence interval [CI] = 8.49% to 14.65%; week 8, 10.21% versus 0.28%, difference = 9.92%, 95% CI = 5.42% to 14.42%; P<.001 for all). However, the average intrapatient libido change from baseline to weeks 4 and 8 was similar on both arms. CONCLUSION: Increased testosterone level did not translate into improved libido, possibly because women on this study were estrogen depleted.