Chronically undernourished patients (n = 10) undergoing elective abdominal surgery were assessed with regard to their energy expenditure and urinary nitrogen loss. These measurements were made for 1 week after the surgery, and stress factors for each parameter were computed. The responses of the chronically undernourished patients were compared to those of relatively well nourished patients (n = 10) undergoing comparable surgeries. It was found that the postoperative resting energy expenditure (REE) of the chronically undernourished patients was not significantly elevated when compared to their preoperative values (mean +/- SEM): 1210.66 +/- 88.13, 1354.91 +/- 86.61, 1215.09 +/- 89.68, and 1188.23 +/- 86.61 kcal/day preoperatively and on postoperative days 1, 4, and 8, respectively. On the other hand, the postoperative REE of the controls was significantly elevated (p < 0.05) over their baseline values: 1357.18 +/- 70.81, 1574.66 +/- 100.35, 1502.89 +/- 109.44, and 1477.23 +/- 83.52; kcal/day, respectively, for the same days. The stress factors for the controls were higher than those for the undernourished (1.16 versus 1.12, 1.11 versus 1.00, and 1.09 versus 0.98 on postoperative days 1, 4, and 8, respectively). The urinary nitrogen excretion in both groups (for the 4 days) was not significantly elevated over baseline (6.23 +/- 0.87, 7.72 +/- 0.71, 8.36 +/- 0.87, and 8.04 +/- 1.56 grams/day in the undernourished; and 7.59 +/- 1.03, 9.57 +/- 1.33, 9.49 +/- 1.03, and 8.67 +/- 0.76 grams/day in the controls. The stress factors for nitrogen excretion were slightly higher in the undernourished group.(ABSTRACT TRUNCATED AT 250 WORDS)