In a study of pancreases from 75 patients who died at presentation of Type I diabetes there was selective destruction of beta cells associated with islet inflammation (insulitis). According to a recent hypothesis, aberrant expression of Class II major histocompatibility complex (MHC) products on a target cell may allow presentation of organ specific surface antigen(s) to potentially autoreactive T helper lymphocytes and thus lead to autoimmunity. Aberrant expression of Class II MHC was demonstrated immunohistochemically on beta cells in 21 out of 23 patients with recent onset diabetes. No such expression was seen on the other pancreatic endocrine cells. Ninety-four per cent of insulin-containing islets in these patients had marked hyperexpressions of Class I MHC affecting all endocrine cells in these islets. Insulin deficient islets were not thus affected. Both these abnormalities of MHC expression appeared to precede insulitis within a given islet and appeared to be unique to Type I diabetes, being absent in pancreases of patients with Type II diabetes, chronic pancreatitis, cystic fibrosis, graft-versus-host disease and Coxsackie B viral pancreatitis. The development of autoimmunity to beta cells in Type I diabetes may be a 'multistep' process in which abnormalities of MHC expression are crucial events.