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Background. Chronic renal replacement therapy patients exhibit reduction in skeletal muscle function as a result of a combination of metabolic effects and muscle fibre size reduction. The aim of this study was to compare muscle mass with function in patients with chronic kidney disease (CKD) at stages 4 and 5 on haemodialysis (HD) and peritoneal dialysis (PD), and investigate the associations of muscle wasting in a cross-sectional cohort. Methods. We studied 134 patients (60 HD, 28 PD and 46 CKD 4). The three groups were well matched for age, sex, diabetes and dialysis vintage. Cross-sectional area (CSA) of muscle and fat was measured from a standardized multi-slice CT scan of a 6 cm long section of thigh. CSA of soft tissue was taken from appropriate fat and muscle densities. Functional assessment was by the sit-to-stand 60 test, assessing both the number of sit-to-stands possible under controlled conditions in 60 s (STS 60), and the time taken to perform five sit-to-stand movements (STS 5). Data were collected on a wide range of potential determinants of muscle CSA.Results. There were no significant differences in haemoglobin between males or females or between any of the groups studied. Serum phosphate and calcium-phosphate product were higher in HD patients as compared to CKD4 patients, but there were no differences in these variables when comparing PD patients with either CKD4 or HD patients. Muscle CSA correlated well with objective functional assessments in males (STS 60 R ¼ 0.52, P<0.0001) and females (R ¼ 0.41, P ¼ 0.004), and STS performance was reduced in dialysed patients as compared with CKD 4. Univariate analysis demonstrated that muscle CSA was associated with serum albumin concentration (R ¼ 0.49, P<0.0001), age (R ¼ À0.35, P ¼ 0.005) and C-reactive protein (R ¼ À0.34, P ¼ 0.004). Creatinine clearance, dialysis adequacy, dialysis vintage and timeaveraged serum bicarbonate, calcium and phosphate concentrations were not correlated with muscle CSA. Conclusion. In conclusion, patients with dialysistreated CKD 5 exhibited more functionally significant muscle wasting than patients with CKD 4. This may be amenable to modification with targeted exercise or amelioration of factors associated with observed differences in muscle mass.