Malnutrition is prevalent in chronic hemodialysis patients and is related to multiple factors; the hemodialysis procedure itself has been suggested as a catabolic factor. To examine the possible role of hemodialysis on energy metabolism, resting energy expenditure and respiratory quotient in ten chronic hemodialysis patients was measured in this study, using a whole-room indirect calorimeter. Measurements were done continuously: for 2 h before hemodialysis, during 4 h of hemodialysis, for 2 h after hemodialysis, and separately on a nondialysis day after 12 h of fasting. Age-, sex-, and body mass index-matched healthy volunteers were used as control subjects. Chronic hemodialysis patients have a significantly higher resting energy expenditure on a nondialysis day (1.18 +/- 0.15 kcal/min; P < 0.01) as compared with control subjects (1.10 +/- 0.16 kcal/ min). Resting energy expenditure further increased significantly during the hemodialysis procedure (1.32 +/- 0.18 kcal/min, averaged over the 4 h of hemodialysis; P < 0.01 versus predialysis) and was also significantly higher compared with the postdialysis period and nondialysis day resting energy expenditure (P < 0.001 for both). This effect was most pronounced during the first (1.37 +/- 0.19 kcal/min) and second (1.33 +/- 0.18 kcal/min) hours of hemodialysis (P < 0.001 for both). Respiratory quotient was not significantly affected by hemodialysis. It was concluded that chronic hemodialysis patients have higher than normal resting energy expenditure levels, which is further increased during hemodialysis. This process may significantly potentiate the protein-calorie malnutrition seen in this patient population.