Malnutrition, shown by fat depletion and muscle wasting, is common in uremia. The methods used to describe body composition in uremia are controversial. We have used ultrasonography of the thigh to evaluate fat and muscle in haemodialysis patients (group 1) (n = 14), peritoneal dialysis patients (group 2) (n = 10) and normal controls (group 3) (n = 13). Musculus rectus femoris diameter (0.84 +/- 0.06 cm versus 1.15 +/- 0.08 cm, p < 0.05) and cross sectional area (3.0 +/- 0.4 cm2 versus 5.5 +/- 0.4 cm2, p < 0.01) were significantly reduced in group 1 compared with group 3, as was also muscle area in group 2 (3.9 +/- 0.6, p < 0.05), indicating malnutrition in the dialysis patients. Peritoneal dialysis patients tended to have more subcutaneous fat (p = 0.07) and a larger musculus rectus diameter (p = 0.09) than haemodialysis patients, possibly reflecting better preserved nutritional status. Ultrasonography was equal to computed tomography for estimating these parameters. Ultrasonography is a useful tool for evaluating body composition in uremia, and has a capacity for detection of malnutrition, and possibly for early discrimination between patient groups with different dialysis modalities.