OBJECTIVE Aging is known to be associated with increasing insulin resistance and declining glucose tolerance. The cause for the insulin resistance, however, remains uncertain. In this study, we examined the hypothesis that at least part of the insulin resistance may be attributable to age-related changes in body composition and muscle blood flow rather than age itself. RESEARCH DESIGN AND METHODS We studied 6 healthy, elderly (66.2 ± 1.7 yr) and 6 younger, healthy men (31.8 ± 3.0 yr) matched for height and weight by determination of their body composition (by underwater weighing), leg blood flow (by mercury strain-gauge plethysmography), rates of glucose uptake (by stable isotope dilution analysis with 6,6 D2-glucose), and carbohydrate oxidation (by indirect calorimetry) during euglycemic hyperinsulinemic clamping. RESULTS Body fat (kg fat mass or in percentage of body weight), rates of insulin-stimulated leg blood flow, glucose uptake, oxidation, and storage were all similar in elderly and younger men. Body fat (in percentage of body weight) of both elderly and younger men correlated closely and negatively with glucose uptake (r = −0.73, P < 0.01), glucose oxidation (r = −0.67, P < 0.05), and with glucose storage (r = −0.65, P < 0.05). In contrast, age did not correlate significantly with any parameter of glucose metabolism. CONCLUSIONS Our data suggested that insulin sensitivity in men until around 60–70 yr of age appears to be determined more by body fat than by age.