Objective The aim of this study was to assess the association between admission weight, weight loss, and length of stay (LOS) in patients with walled-off pancreatic necrosis. Methods We classified the admission body mass index (BMI) of 18.5 to <25 kg/m² as normal weight, 25 to <30 kg/m² as overweight, and ≥30 kg/m² as obesity. The Nutritional Risk Screening score-2002 was calculated to identify patients at risk for undernutrition. Results We included 38 patients (61% men, 68% with infected necrosis; 40% normal weight; 60% overweight/obesity). Four patients (11%) required treatment at the semi-intensive care unit, 11 (29%) developed pneumonia, and 10 (26%) developed septicemia. One patient died due to respiratory failure and hemorrhage. The remaining patients were discharged after a median of 49 d (36–64 d). During admission, 14 patients (38%) achieved an energy–protein intake of at least 75% and 17 (46%) achieved ≥70% coverage. The percentage weight loss was different (P < 0.01) for patients with normal weight (4%), overweight (9%), and obesity (14%). There was no difference between groups regarding percentage of energy or protein coverage. Patients with overweight/obesity had a longer hospital LOS (P = 0.016). In univariable regression analysis, overweight, obesity, energy, and protein coverage predicted weight loss. LOS did not predict weight loss. In multivariable regression analysis, overweight and obesity were the only remaining significant predictors of weight loss. Conclusions Patients with walled-off pancreatic necrosis are at considerable risk for undernutrition. A BMI >25 kg/m² predicts greater weight loss and longer LOS.