 This is a prospective pilot study of 50 patients with first attack of moderately severe or severe acute pancreatitis AP classified by the new revised criteria for AP 2012 treated in Helsinki University Hospital. The included patients are treated according to the existing hospital practice and current guidelines for AP American Pancreatic Association guidelines. The development of pancreatic exocrine insufficiency is determined by measuring the fecal elastase-1 test. The test is measured during the hospitalization time baseline testing, and in the follow-up period 1-2 months and 12 months after discharging from hospital. The baseline test will be measured as soon as the severity of AP is known and patient is included in the study, and the first stool sample is available. Patients with previous history of AP, chronic pancreatitis, known pancreatic exocrine insufficiency or previous pancreatic surgery will be excluded. 