Among 88 unselected patients with chronic pancreatitis 35% (95% confidence limits 25 to 46) had insulin-dependent diabetes, 31% (21% to 41%) had non-insulin-dependent diabetes or impaired glucose tolerance (by intravenous glucose tolerance test), and 34% (24% to 45%) had normal glucose tolerance. B cell function measured by C-peptide concentration after 1 mg glucagon IV correlated with the pancreatic enzyme secretion (meal stimulated duodenal lipase content). B cell function was preserved to a greater extent (P less than .01), and glycosylated hemoglobin and fasting level of glucose were lower (P less than .01 to .05) in the 31 patients with pancreatogenic diabetes than than in 35 otherwise comparable patients with type I (insulin-dependent) diabetes, yet daily insulin dose was similar in the two groups. Glucagon stimulated C-peptide was inversely correlated to glycosylated hemoglobin in insulin-dependent patients with pancreatogenic diabetes and in type I diabetes. Since body mass indices were identical in the two groups, better glucoregulation was not due to reduced food intake or malabsorption in pancreatogenic diabetes. Rather residual B cell function and/or different secretion of other pancreatic hormones in pancreatogenic diabetes may account for different metabolic control in type I IDDM compared with insulin-dependent pancreatogenic diabetes.