Under basal resting conditions muscle metabolism is reduced, whereas metabolism increases with physical activity. We wished to determine whether there was an association between resting energy expenditure (REE) and total energy expenditure (TEE) in peritoneal dialysis (PD) patients and lean body mass (LBM). We determined REE and TEE by recently validated equations, using doubly labelled isotopic water, and LBM by dual-energy X-ray absorptiometry (DXA) scanning. We studied 87 patients, 50 male (57.4%), 25 diabetic (28.7%), mean age 60.3 ± 17.6 years, with a median PD treatment of 11.4 (4.7–29.5) months. The mean weight was 70.1 ± 17.7 kg with a REE of 1509 ± 245 kcal/day and TEE 1947 ± 378 kcal/day. REE was associated with body size (weight r = 0.78 and body mass index (BMI) r = 0.72) and body composition (LBM r = 0.77, lean body mass index (LBMI) r = 0.76, r = 0.62), all P<0.001). For TEE, there was an association with weight r = 0.58, BMI r = 0.49 and body composition (LBM r = 0.64, LBMI (r = 0.54), all P<0.001). We compared LBMI measured by DXA and that estimated by the Boer equation using anthropomorphic measurements, which overestimated and underestimated LBM for smaller patients and heavier patients, respectively. Muscle metabolism is reduced at rest and increases with physical activity. Whereas previous reports based on REE did not show any association with LBM, we found an association between both REE and TEE, using a recently validated equation derived from dialysis patients, and LBM measured by DXA scanning. Estimation of muscle mass from anthropomorphic measurements systematically overestimated LBM for small patients and conversely underestimated for heavier patients.