Background: Resting energy expenditure (REE) is increased in patients with cystic fibrosis (CF) with end-stage lung disease due to chronic inflammation and pulmonary infections. After lung transplantation (LTx), energy expenditure is expected to be lower because inflammation will decrease. We assessed the agreement between measured and predicted REE in pre-LTx CF and post-LTx patients with CF and differences in REE in pre-LTx CF and post-LTx patients with CF in a cross-sectional study. Methods: Included were 12 pre-LTx patients with CF (9 women; median age 31.6 years; interquartile range [IQR], 23.3–40.0) and 12 patients with CF within 2 years after LTx (6 women; median age 33.5 years; IQR, 22.3–40.3). REE was measured in a fasted state using indirect calorimetry. Values were compared with predicted REE calculated by formulas of Harris-Benedict (1919 and 1984), Schofield, and the World Health Organization (1985). A calculated REE between 90% and 110% of REE measured was considered adequate. Results: Prediction equations underestimate REE in at least 75% of pre-LTx and 33% of post-LTx patients with CF. Mean (SD) REE measured by indirect calorimetry was 1735 (251) kcal pre-LTx and 1650 (235) kcal post-LTx (P = .40). REE expressed per kilogram of fat-free mass (FFM) was 40.5 kcal/kg in pre-LTx patients with CF, which was higher than the 34.3 kcal/kg in post-LTx patients with CF (P = .01). Conclusions: Prediction equations underestimate REE in patients with end-stage CF. REE per kg of FFM is lower post-LTx than pre-LTx in patients with CF. Measurement of REE is recommended for patients with CF, especially pre-LTx, to optimize energy requirements for improving nutrition status.