Quantitative morphometry of immunostained pancreatic islet cells in a surgical specimen from a maturity-onset diabetic (MOD) patient (A) has been combined with a study of beta-cell secretion in him and his MOD son (B). The morphometry showed intra-islet amyloid deposits in 24% of islet sections which is a similar distribution to that found in 7 other MODs, (median 24%, range 0-95%). The distribution of islet cells in the pancreas from A was similar to the 7 MODs and 9 age matched non-diabetic subjects (mean beta-cell area per exocrine area, A, 2%; MODs, 1.45%; controls 1.8%). There was a significantly smaller percentage beta-cell area per islet area in A and the other MODs compared to the controls. Beta-cell secretion was studied by an intravenous glucose infusion, 5 mg/kg ideal body wt. min-1 in A and B and 4 other MODs on sulphonylurea or diet therapy. Fasting plasma glucose was high in both A and B (16 and 7.6 mmol/l respectively) in spite of sulphonylurea therapy. Similarly, A and B had elevated fasting plasma levels of insulin, (0.055 and 0.13 pmol/ml respectively) and C-peptide (0.573 and 1.39 pmol/ml respectively). Plasma proinsulin formed a higher percentage of the immunoreactive insulin in fasting plasma samples of both A (36%) and B (43%) than in less hyperglycaemic MODs on diet alone (26%) or sulphonylurea therapy (17%) and the glucose stimulated proinsulin content was even higher (A, 50%; B, 53%; MODs, diet, 19%; sulphonylureas, 16%). Elevated proinsulin levels in a patient with islet-amyloid is consistent with the hypothesis that amyloid may be derived from abnormal beta-cell secretion.