Context:Studies of the possible cardiovascular risk of testosterone treatment are inconclusive. Objective:To determine the effect of testosterone treatment on cardiovascular biomarkers in older men with low testosterone. Design:Double-blind, placebo-controlled trial. Setting:Twelve academic medical centers in the United States. Participants:In all, 788 men ≥65 years old with an average of two serum testosterone levels <275 ng/dL who were enrolled in The Testosterone Trials. Intervention:Testosterone gel, the dose adjusted to maintain the testosterone level in the normal range for young men, or placebo gel for 12 months. Main Outcome Measures:Serum markers of cardiovascular risk, including lipids and markers of glucose metabolism, fibrinolysis, inflammation, and myocardial damage. Results:Compared with placebo, testosterone treatment significantly decreased total cholesterol (adjusted mean difference, -6.1 mg/dL; P < 0.001), high-density lipoprotein cholesterol (adjusted mean difference, -2.0 mg/dL; P < 0.001), and low-density lipoprotein cholesterol (adjusted mean difference, -2.3 mg/dL; P = 0.051) from baseline to month 12. Testosterone also slightly but significantly decreased fasting insulin (adjusted mean difference, -1.7 µIU/mL; P = 0.02) and homeostatic model assessment‒insulin resistance (adjusted mean difference, -0.6; P = 0.03). Testosterone did not change triglycerides, d-dimer, C-reactive protein, interleukin 6, troponin, glucose, or hemoglobin A1c levels more than placebo. Conclusions and Relevance:Testosterone treatment of 1 year in older men with low testosterone was associated with small reductions in cholesterol and insulin but not with other glucose markers, markers of inflammation or fibrinolysis, or troponin. The clinical importance of these findings is unclear and requires a larger trial of clinical outcomes.