The level of elastase 1 in feces was markedly decreased in all patients with tumors of the pancreatic head or its involvement before and after gastropancreatoduodenal resection and was close to normal in patients with retroperitoneal tumors not invading the pancreas. Serum elastase 1 concentrations were virtually the same in patients, normal subjects, and patients without signs of pancreatic involvement. The presence of elastase 1 in feces reflects adequate function of the pancreato-digestive anastomosis, while low concentrations of the enzyme indicate impaired function of the pancreas, presumably because of operation trauma and tumor process. Measurement of elastase 1 in feces is a highly informative and specific test for evaluation of pancreatic function in patients with pancreatic tumors, which can be used in clinical practice.