Following a recommendation by the Institute of Medicine, the authors sought to garner reliable evidence regarding the beneficial effects and the safety of testosterone—a therapy that's widely used in older men who show low levels on a serum testosterone test. . This randomized double-blind study of one year of testosterone treatment was funded by the National Institutes of Health. It involved multiple, overlapping populations and various statistical approaches in a coordinated set of trials. Raising testosterone concentrations in the study population, 788 men, was associated with moderate improvements in sexual function and with small positive effects on mood and depressive symptoms. But there were no statistically significant benefits on the men's vitality or their six-minute walking distance. The sample size was selected to assess potential beneficial effects and will inform related clinical decision-making; the size was inadequate to assess the risk of cardiovascular events or prostate cancer.