Isolated amyloid from the islets of Langerhans of patients with maturity onset diabetes mellitus was compared with amyloid fibrils from patients with different types of systemic amyloidosis. It was found that systemic amyloids had in common rigid and non-branching filaments with a width of about 75 A and that these filaments sometimes were attached laterally, forming thicker fibrils. Similar filaments could also be extracted from islet amyloid but the main part of this amyloid was built up by large aggregates of very thin and often very wavy units. This structure, which has not been previously described in human amyloid, probably explains some properties of isolated islet amyloid.