To determine if oxygen consumption (VO2) derived from the Fick equation (FE) can be used to determine energy expenditure (EE), 29 paired indirect calorimetry (IC) and FE VO2 determinations were obtained. The Weir equation was used to calculate EE from the FE VO2 value. There was a strong correlation between the methods (r = 0.82, p less than 0.001). Mean EE by IC and FE was 2460 +/- 539 and 2372 +/- 787 kcal/day, respectively, a difference of 88 +/- 467 kcal/day. A single IC determination is often used to guide nutrition for several days. To evaluate this practice, FE and IC determinations were repeated in 8 patients. There was a 19% difference in EE between initial and follow-up IC, which was identical to the mean difference between paired FE and IC measurements. FE can be used to estimate EE, and is as accurate as using a single IC reading to predict EE on subsequent days.