Whether lean body mass (LBM) composition, especially skeletal muscle and abdominal organs, differs in adults with type 2 diabetes (T2DM) compared with nondiabetic healthy controls has not been investigated. A subset of African-American and Caucasian participants with T2DM from the Look AHEAD (Action for Health in Diabetes) trial had body composition assessed and compared with a sample of healthy controls. Skeletal muscle mass (SMM), liver, kidneys, and spleen mass were quantified using a contiguous slice magnetic resonance imaging (MRI) protocol. Cardiac mass was quantified by either a cardiac gated MRI protocol or by echocardiography. MRI volumes were converted to mass using assumed densities. Dual-energy X-ray absorptiometry assessed LBM. Using general linear models adjusted for height, weight, sex, age, race, and interactions of diabetes status with race or sex, persons with T2DM (n ϭ 95) had less LBM (49.7 vs. 51.6 kg) and SMM (24.1 vs. 25.4 kg) and larger kidneys (0.40 vs. 0.36 kg) than controls (n ϭ 76) (all P Ͻ 0.01). Caucasians with T2DM had larger livers (1.90 vs. 1.60 kg, P Ͻ 0.0001) and spleens (0.29 vs. 0.22 kg, P Ͻ 0.01), and T2DM men had less cardiac mass than controls (0.25 vs. 0.30 kg, P Ͻ 0.001). In this sample, T2DM is characterized by less relative skeletal muscle and cardiac mass in conjunction with larger kidneys, liver, and spleen. Further investigation is needed to establish the causes and metabolic consequences of these race-and sex-specific organ mass differences in T2DM. African American; Caucasian; magnetic resonance imaging; kidneys; liver; spleen THE CHARACTERIZATION OF BODY composition in persons with type 2 diabetes mellitus (T2DM) has been largely limited to fat distribution and its effects on metabolic disease (2,3,23,24,40,(43)(44)(45). Because skeletal muscle mass is the major site of glucose disposal and insulin uptake (52), it is of clinical importance to know whether this metabolically important tissue, which constitutes ϳ75% of total body lean mass in healthy adults (46), is preserved in persons with T2DM. Longitudinal data in older adults indicate that the age-associated reduction in thigh cross-sectional area and appendicular lean mass is accelerated in women with T2DM compared with healthy controls, but this difference was not found in men (35). Another possible contributor to the apparent differences in the quantity of lean tissue mass lost in individuals with T2DM may be the effect of race. Recent evidence links the established energy expenditure differences between African Americans and Caucasians (18, 29) to discrepancies in high-metabolic-rate organ masses such as liver, kidneys, heart, spleen, and brain (15). Whether these sex-and race-dependent effects on skeletal muscle or organ mass affect lean tissue preservation in persons with diabetes, a disease state that directly affects protein metabolism (19,37) and may influence the relative size of lean tissues and organs, is still unknown.The primary aim of this study was to assess lean body mass (LBM), skeletal muscl...