BACKGROUND Few long-term randomized trials have evaluated the efficacy of testosterone replacement therapy (TRT) in improving sexual function and hypogonadal symptoms in men with hypogonadism and whether effects are sustained beyond 12 months. OBJECTIVE The Testosterone Replacement therapy for Assessment of long-term Vascular Events and efficacy ResponSE in hypogonadal men (TRAVERSE) study evaluated the effect of TRT on major adverse cardiovascular events in middle-aged and older men with hypogonadism. The Sexual Function Study, nested within the parent trial, determined testosterone's efficacy in improving sexual activity, hypogonadal symptoms, libido, and erectile function among men reporting low libido. METHODS Among 5204 men, 45-80 years, with 2 testosterone concentrations <300 ng/dL, hypogonadal symptoms, and cardiovascular disease (CVD) or increased CVD risk enrolled in the TRAVERSE trial, 1161 with low libido were enrolled in the Sexual Function Study (587 randomized to receive 1.62% testosterone gel and 574 to placebo gel for the duration of their participation in the study). Primary outcome was change from baseline in sexual activity score. Secondary outcomes included hypogonadal symptoms, erectile function, and sexual desire. RESULTS TRT was associated with significantly greater improvement in sexual activity than placebo (estimated mean [95% CI] between-group difference 0.49 [0.19,0.79] and 0.47 [0.11, 0.83] acts per day at 6 and 12 months, respectively; omnibus test P = .011); treatment effect was maintained at 24 months. TRT improved hypogonadal symptoms and sexual desire, but not erectile function, compared with placebo. CONCLUSIONS In middle-aged and older men with hypogonadism and low libido, TRT for 2 years improved sexual activity, hypogonadal symptoms, and sexual desire, but not erectile function.