Background: While the long-term survival rate among patients with pancreatic and periampullary carcinomas remains low, it can be influenced by various factors. The purpose of this retrospective study was to investigate the effects of body mass index (BMI) on postoperative complications and patient survival after pancreatic resections for underlying malignancy over a 20-year observation period. Methods: We analyzed 1,384 patients, 918 patients with pancreatic ductal adenocarcinoma (PDAC) (66.3％), 229 patients with distal cholangiocarcinoma (16.5％), 206 ampullary carcinoma patients (14.8％), and 31 duodenal carcinoma patients (2.2％). Patients were classified into four groups (group 1 30.0) according to their BMI (kg/m2). We analyzed differences in postoperative complications, postoperative length of hospital stays, reoperations, postoperative mortality and survival rate among the groups. Results: Within a mean observation period of 687.7 [2–8,500] days, 735 (53.1％) patients died. There were important differences in postoperative complications (group 1, 16.2％; group 2, 20.3％; group 3, 27.2％, group 4, 41.6％) with the type of postoperative complications also varying between the groups. Overall 1-, 5-, 10- and 15-year survival rates were 66.4％, 25.5％, 17.9％, and 12.1％, respectively, with survival rates varying amongst the four groups. Conclusions: Patients with a BMI between 18.5 and 30 show better postoperative outcomes, regarding complications, hospitalization duration, and reoperation rates than underweight or obese patients. Short-term survival depends strongly on postoperative complications while patients with a higher BMI show better long-term survival rates.