To determine whether there was an association between the size of the pancreas and the type of diabetes, ultrasonography of the pancreas was performed on 57 diabetic patients: 14 with Type 1 (insulin-dependent) diabetes, 10 insulin-treated and 33 tablet-treated patients with Type 2 (non-insulin-dependent) diabetes, and 19 non-diabetic subjects. The pancreas of patients with Type 1 diabetes was markedly smaller (p < 0.0001) than the pancreas in non-diabetic subjects. The pancreas of patients with Type 2 diabetes was more moderate in size: larger (p < 0.001) than that of Type 1 diabetic patients but smaller (p < 0.5) than the pancreas of the control group. Pancreatic size of patients with Type 2 diabetes was also related to basal insulin secretion with insulin-deficient patients (low or undetectable C-peptide) having smaller (p < 0.05) pancreases than those with normal insulin secretion. There was no difference in the size of the pancreas in the different treatment groups of Type 2 diabetic patients. Pancreatic size did not correlate with age, body mass index or the duration of diabetes. We conclude that the pancreas is a smaller organ in patients with diabetes mellitus and that the decrement in size is maximal in insulin-dependent/insulin-deficient subjects. Ultrasonography, therefore, can potentially serve to discriminate between the different types of diabetes.