Fibrillar protein deposits (amyloid) in the pancreatic islets of Langerhans are thought to be involved in death of the insulinproducing islet ␤ cells in type 2 diabetes mellitus. It has been suggested that the mechanism of this ␤ cell death involves membrane disruption by human islet amyloid polypeptide (hIAPP), the major constituent of islet amyloid. However, the molecular mechanism of hIAPP-induced membrane disruption is not known. Here, we propose a hypothesis that growth of hIAPP fibrils at the membrane causes membrane damage. We studied the kinetics of hIAPP-induced membrane damage in relation to hIAPP fibril growth and found that the kinetic profile of hIAPP-induced membrane damage is characterized by a lag phase and a sigmoidal transition, which matches the kinetic profile of hIAPP fibril growth. The observation that seeding accelerates membrane damage supports the hypothesis. In addition, variables that are well known to affect hIAPP fibril formation, i.e., the presence of a fibril formation inhibitor, hIAPP concentration, and lipid composition, were found to have the same effect on hIAPP-induced membrane damage. Furthermore, electron microscopy analysis showed that hIAPP fibrils line the surface of distorted phospholipid vesicles, in agreement with the notion that hIAPP fibril growth at the membrane and membrane damage are physically connected. Together, these observations point toward a mechanism in which growth of hIAPP fibrils, rather than a particular hIAPP species, is responsible for the observed membrane damage. This hypothesis provides an additional mechanism next to the previously proposed role of oligomers as the main cytotoxic species of amyloidogenic proteins.amylin ͉ amyloid cytotoxicity ͉ large unilamellar vesicles ͉ protein-membrane interaction ͉ type 2 diabetes mellitus T ype 2 diabetes mellitus (DM2) is characterized histopathologically by the presence of fibrillar amyloid deposits in the pancreatic islets of Langerhans. Amyloid cytotoxicity is thought to be an early mechanism involved in death of insulin-producing islet ␤ cells in DM2 (1). The main component of islet amyloid, and the actual fibril-forming molecule, is a 37-amino acid peptide called human islet amyloid polypeptide (hIAPP) or amylin, which is produced together with insulin in the pancreatic islet ␤-cells. It is thought that ␤ cells of DM2 patients are somehow killed through hIAPP-induced damage of the ␤ cell membrane (2). However, our knowledge of the mechanism of hIAPP-induced membrane damage is extremely sparse. It is not known how cytotoxic hIAPP species interact with cellular membranes and induce cell death. Furthermore, it is not established whether cytotoxic hIAPP species are formed before contacting the membrane or whether a membrane environment is in fact required for the formation of cytotoxic hIAPP species.The prevailing view is that membrane damage and concomitant ␤ cell death are caused by cytotoxic hIAPP oligomers (2-9). There are indications that these oligomers form ion channels (2, 3), as has been suggeste...