Evaluation of the safety of hormonal preparations for the treatment of female sexual dysfunction is important to assess the benefit-to-risk profile of these drugs and has been strongly encouraged by the Food and Drug Administration. LibiGel (Biosante Pharmaceuticals, Inc., Lincolnshire, IL), a low-dose testosterone gel, is under development for the treatment of hypoactive sexual desire disorder (HSDD) in oophorectomized women. To evaluate the long-term effects of LibiGel on risk for cardiovascular (CV) events, breast cancer, and general safety, a randomized, placebo-controlled clinical study using a novel adaptive design to optimize sample size and power is being conducted. The primary end point of the BioSante LibiGel Safety Study (BLISS) is a composite of CV events including death, nonfatal myocardial infarction, nonfatal stroke, coronary revascularization, hospitalized unstable angina, and venous thromboembolic events. Breast cancer is a coprimary end point. Postmenopausal women (both surgically and naturally) with HSDD and increased risk for CV events will be followed up for up to 5 years postrandomization with an interim data analysis for regulatory approval after the last woman enrolled has been on therapy for at least 12 months. Determination of the number of subjects to enroll is based on an adaptive design that uses interim data to estimate the predictive probability of study success. In agreement with the Food and Drug Administration, LibiGel will be declared safe if the upper limit of the 97.2% CI of the hazard ratio is ≤2.0 or the upper bound of the 97.2% CI for the absolute difference between CV event rates per 100 person-years is ≤1% and the observed hazard ratio is ≤2.0. The BLISS study will define the CV safety profile of low-dose testosterone therapy in the formulation of LibiGel for postmenopausal women with HSDD, and the trial design may provide a paradigm for studies that aim to document long-term safety when the proposed outcome under study is an uncommon adverse event.