Several proteins have been identified as amyloid forming in humans, and independent of protein origin, the fibrils are morphologically similar. Therefore, there is a potential for structures with amyloid seeding ability to induce both homologous and heterologous fibril growth; thus, molecular interaction can constitute a link between different amyloid forms. Intravenous injection with preformed fibrils from islet amyloid polypeptide (IAPP), proIAPP, or amyloid-beta (Aβ) into human IAPP transgenic mice triggered IAPP amyloid formation in pancreas in 5 of 7 mice in each group, demonstrating that IAPP amyloid could be enhanced through homologous and heterologous seeding with higher efficiency for the former mechanism. Proximity ligation assay was used for colocalization studies of IAPP and Aβ in islet amyloid in type 2 diabetic patients and Aβ deposits in brains of patients with Alzheimer disease. Aβ reactivity was not detected in islet amyloid although islet β cells express AβPP and convertases necessary for Aβ production. By contrast, IAPP and proIAPP were detected in cerebral and vascular Aβ deposits, and presence of proximity ligation signal at both locations showed that the peptides were <40 nm apart. It is not clear whether IAPP present in brain originates from pancreas or is locally produced. Heterologous seeding between IAPP and Aβ shown here may represent a molecular link between type 2 diabetes and Alzheimer disease.