Although short-term administration of testosterone in replacement doses is relatively safe, the risks of long-term testosterone administration in older men remain poorly understood. The two major areas of concern include the potential for increased risk of atherosclerotic heart disease and exacerbation of a pre-existing, subclinical prostate cancer. There is a widespread perception that testosterone supplementation adversely affects plasma lipoprotein profile and increases the risk of atherosclerotic heart disease; this premise is not supported by data. Thus, the long-term consequences of testosterone supplementation on the risk of atherosclerosis progression remain unknown. While supraphysiological doses of testosterone and non-aromatizable androgens frequently employed by body-builders undoubtedly decrease plasma high density lipoprotein (HDL)-cholesterol levels, physiologic testosterone replacement in older men has been associated with only a modest or no decrease in plasma HDL-cholesterol. Cross-sectional studies of middle-aged men find a direct, rather than an inverse, relationship between serum testosterone levels and plasma HDL-cholesterol concentrations as well as an inverse correlation between serum testosterone levels and visceral fat volume. Testosterone supplementation of middle-aged men with truncal obesity is associated with a reduction in visceral fat volume, serum glucose concentration, blood pressure, and an improvement in insulin sensitivity. All of these changes are associated with lower risk for atherosclerosis. These data suggest that serum testosterone levels in the range that is mid-normal for healthy young men are consistent with an optimal cardiovascular risk profile at any age, and that testosterone concentrations either above or below the physiologic male range may increase the risk of atherosclerotic heart disease. Studies in a low density lipoprotein (LDL)-receptor deficient mice provide compelling evidence that testosterone retards early atherogenesis, and that testosterone effects on atherogenesis are mediated through its conversion to estradiol by the action of aromatase enzyme that is expressed in the vessel wall. The effects of testosterone replacement on cardiovascular risk in humans have never been directly examined. Therefore, the primary objective of this study is to examine directly the effects of testosterone replacement on atherosclerosis progression in men by measuring common carotid artery intima-media thickness (CCA IMT) and coronary artery calcification (CAC) by multidetector computed tomography (MDCT), two independent measurements of generalized atherosclerosis.