A possible immunogenetic basis for diabetes in chronic pancreatitis was explored by studying 19 patients with both disorders, most of whom required treatment with insulin. In contrast to patients with insulin-dependent (Type 1) diabetes, patients with diabetes and chronic pancreatitis had residual beta cell function but blunted C-peptide responses to intravenous glucagon, absence of circulating islet cell antibodies, and HLA-DR types similar to control subjects and patients with chronic pancreatitis without diabetes. Diabetes complicating chronic pancreatitis is therefore not associated with the biochemical or immunogenetic markers characteristic of Type 1 diabetes.