Resting energy expenditure (REE) was measured in 165 malnourished patients with human immunodeficiency virus infection and compared with that of 31 control subjects. Of these patients, 129 had no symptoms of secondary infections (NI), whereas 36 had evolving secondary infection (I) despite a body temperature < 38 degrees C. Mean REE was 11% higher in the NI group than in the control group (P < 0.05); it was 34% higher in the I group than in the control group (P < 0.001) and 21% higher than in the NI group (P < 0.01). In all cases the best predictive variable for REE was the fat-free mass (FFM) (r = 0.77, P < 0.001 in NI; r = 0.70, P < 0.001 in I; r = 0.78 in the control group. The REE-FFM ratio was 152.5 +/- 1.2 and 207 +/- 5.4 kJ.kg-1.d-1 in the NI and I groups, respectively. A high energy expenditure may be a harbinger of secondary infection in acquired immunodeficiency syndrome and may participate in accelerated weight loss.