Ultrastructural alterations in pancreatic acini from six patients operated for acute necrotizing pancreatitis are described. One of the patients suffered from biliary tract disease, the rest had excessive alcohol intake as the presumed aetiology. Areas of the pancreatic parenchyma showing oedematous inflammation in light microscopy were studied in the electron microscope. Findings in acinar cells included changes in zymogen granules and an increased autophagocytosis in addition to unspecific organelle alterations. Zymogen granules showed increase in size and number, loss or variation of electron-density and peripheral dissolution. Increased autophagic activity was indicated by several autophagic vacuoles and residual bodies. Acinar lumina were dilated showing effacement of microvilli and invaginations in the luminal plasma membrane of the acinar cells. In acinar lumina and in the interstitium fibrillar material was observed, with an increasing frequency in those areas showing severe cellular disintegration. These findings suggest: 1) an increased activity of zymogen granules, 2) an increased autophagocytosis, and 3) penetration of acinar luminal contents into the interstitium.