Objective: Ensuring adequate nutritional support in patients with walled-off pancreatic necrosis (WON) is challenging and weight loss is often considerable. The aim of this study was to evaluate resting energy expenditure (REE) and body composition in patients with WON. Methods: We prospectively included 18 patients (67% men; median age 63 y; 44% gallstones; 39% alcohol) with WON undergoing endoscopic transgastric drainage and necrosectomy. Patients were followed for 4 wk after admission. We assessed hand-grip strength, REE using indirect calorimetry, and body composition with dual-energy x-ray absorptiometry to assess the percentage change in muscle mass (MM) and fat mass (FM). Data are summarized using medians (range). Results: At baseline, the median body mass index was 27.9 kg/m² (17.7–35.6 kg/m²). Fifteen patients (83%) had infected WON. Eight patients (44%) received total or supplemental parenteral nutrition. The median percentage loss in MM was 0.31% and FM was 6.2%. The median REE was 6870 kJ (3255–8870 kJ) at baseline. Compared with the predicted REE, the measured REE was 1049 kJ higher (–3065 to 2126 kJ) at baseline and –951 kJ lower (–2600 to 3202 kJ) at 4 wk. The difference between the predicted and measured REE at baseline was correlated with the percentage loss in MM (P = 0.043) and FM (P = 0.026). Additionally, patients with infected WON had significantly higher REE (P = 0.003). Conclusion: In patients with WON, an increased REE appears to predict increased muscle and fat loss. Additional studies are necessary to evaluate if REE may be used to improve nutritional support.