Obesity is associated with increased lean mass but its effects on lean-tissue density are less clear. To examine the effects of obesity and non-insulin-dependent diabetes mellitus (NIDDM) on lean-tissue composition and density, cross-sectional computed tomography (CT) scans of the midthigh were obtained in 20 men of various weights. Obesity was associated with increases in thigh-adipose (r = 0.75) and lean-tissue volumes (r = 0.52) and with reduced density of lean tissue (r = -0.73). The increased lean tissue in obesity was due to a nonadipose tissue component with a density below the normal range of muscle, an effect compounded by NIDDM, whereas normal-density muscle volume was unchanged.