A double-blind, randomized, placebo-controlled trial investigating the effect of testosterone supplementation in older men involved 237 healthy male subjects 60–80 years of age with a testosterone level lower than 13.7 nmol/l (in 71% below 12.0 nmol/l). Participants received either 80 mg of testosterone undecenoate or placebo twice daily for 6 months. 207 men completed the study. Lean body mass increased (change difference 1.2 kg, 95% CI 0.7 to 1.7) and fat mass decreased (change difference −1.3 kg, 95% CI −1.8 to −0.8) in the testosterone group compared with the placebo group but this was not accompanied by an increase of functional mobility or muscle strength. Cognitive function (8 different cognitive instruments) and bone mineral density did not change. Insulin sensitivity improved but high-density lipoprotein cholesterol decreased; by the end of the study, 47.8% in the testosterone group vs. 35.5% in the placebo group had the metabolic syndrome (P=0.07). At the same time, total cholesterol decreased significantly in the testosterone group (change difference −0.2, 95% CI −0.4 to 0). Quality-of-life measures were no different except for one hormone-related quality-of-life measure that improved. No negative effects on prostate safety were detected.