INTRODUCTION: The change in aetiology over time of acute and chronic pancreatitis has been sparsely described, as has also the validity of the diagnostic codes. The aim of the study was 1) to clarify whether the aetiology of acute and chronic pancreatitis changed during the period 1983-2005, and 2) to validate the diagnostic codes over time for acute and chronic pancreatitis registered in the Danish National Patient Registry (NPR) in the same period. MATERIAL AND METHODS: All admissions at Hvidovre Hospital coded in the NPR in 1983, 1994 and 2005 with a diagnosis of either acute or chronic pancreatitis were included. After exclusion of readmissions, the cohorts consisted of 92, 146 and 118 patients, respectively. Medical records from every admission were retrieved, the aetiology was assessed and the coding of the diagnoses was related to internationally approved criteria. RESULTS AND CONCLUSION: Gallstone disease significantly (p = 0.04) increased as the cause of acute pancreatitis over the 22-year period, while alcohol remained the major cause of chronic pancreatitis. The validity of the diagnoses for patients with acute pancreatitis varied between 51% and 73%, and for chronic pancreatitis between 63 and 78%.