Context: Impairments in the pituitary-gonadal axis with aging are associated with loss of muscle mass and function and accumulation of upper body fat. Objectives: We tested the hypothesis that physiological supplementation with testosterone and GH together improves body composition and muscle performance in older men. Design, Setting, and Participants: One hundred twenty-two community-dwelling men 70.8 ± 4.2 yr of age with body mass index of 27.4 ± 3.4 kg/m2, testosterone of 550 ng/dl or less, and IGF-I in lower adult tertile (≤167 ng/dl) were randomized to receive transdermal testosterone (5 or 10 g/d) during a Leydig cell clamp plus GH (0, 3, or 5 μg/kg·d) for 16 wk. Main Outcome Measures: Body composition by dual-energy x-ray absorptiometry, muscle performance, and safety tests were conducted. Results: Total lean body mass increased (1.0 ± 1.7 to 3.0 ± 2.2 kg) as did appendicular lean tissue (0.4 ± 1.4 to 1.5 ± 1.3 kg), whereas total fat mass decreased by 0.4 ± 0.9 to 2.3 ± 1.7 kg as did trunk fat (0.5 ± 0.9 to 1.5 ± 1.0 kg) across the six treatment groups and by dose levels for each parameter (P ≤ 0.0004 for linear trend). Composite maximum voluntary strength of upper and lower body muscles increased by 14 ± 34 to 35 ± 31% (P < 0.003 in the three highest dose groups) that correlated with changes in appendicular lean mass. Aerobic endurance increased in all six groups (average 96 ± 137sec longer). Systolic and diastolic blood pressure increased similarly in each group with mean increases of 12 ± 14 and 8 ± 8 mm Hg, respectively. Other predictable adverse events were modest and reversible. Conclusions: Supplemental testosterone produced significant gains in total and appendicular lean mass, muscle strength, and aerobic endurance with significant reductions in whole-body and trunk fat. Outcomes appeared to be further enhanced with GH supplementation.