Malnutrition and wasting are important determinants of morbidity and mortality in patients with chronic renal failure on dialysis. The aim of this study was to determine body composition and energy metabolism in patients with chronic renal insufficiency before dialysis. We compared 15 patients (9 women and 6 men) with chronic renal failure (creatinine, 1.5 to 4.2 mg/dL) with 15 normal subjects pair-matched for sex, age (renal failure versus normal, 71 ± 3 years versus 64 ± 3 years), height (1.61 ± 0.02 m versus 1.64 ± 0.02 m), and weight (64.5 ± 2.7 kg versus 66.4 ± 1.5 kg). Body composition was measured by dual-energy x-ray absorptiometry, and total body water was measured by bioelectrical impedance. Energy metabolism was determined by indirect calorimetry. The average glomerular filtration rate for the patients with chronic renal insufficiency was 23.9 ± 2.6 mL/min/1.73 m2. Lean body mass (41.1 ± 2.0 kg versus 44.5 ± 2.2 kg; P = 0.003) and bone mineral content (2.35 ± 0.11 kg versus 2.72 ± 0.12 kg; P = 0.007) were significantly lower in chronic renal insufficiency; however, fat body mass was the same (19.9 ± 2.1 kg versus 19.1±1.4 kg; P = 0.68). Total body water was similar in renal failure (33.4 ± 1.5 L versus 34.4 ± 1.3 L; P = 0.13). Basal energy expenditure was significantly lower in chronic renal insufficiency (1,085 ± 50 kcal/24 hours versus 1,280 ± 54 kcal/24 hours; P = 0.02), even after adjustment for the differences in lean body mass. Daily caloric intake indicated energy intake was similar in the patients with chronic renal insufficiency and the controls. Patients with a relatively modest degree of chronic renal insufficiency are characterized by reduced lean body mass, bone mineral content, and basal energy expenditure. The determinants of lean body mass in chronic renal insufficiency require further investigation.