The intravenous insulin tolerance test (ITT) allows general assessment of insulin sensitivity by determining the fall in plasma glucose after injection of 0.1 unit/kg regular insulin. To evaluate the usefulness of this test diagnostically in distinguishing Type I from Type II diabetes, 24 patients with Type I diabetes (defined as early age of onset, ideal body weight or less at onset, and insulin dependence) underwent ITTs. Seven patients had normal glucose disposal rates (Ki greater than or equal to 2.5. Nine patients had Ki less than 2.5 but greater than 1.0. Eight patients had Ki less than 1.0. The slopes did not correlate with the control of the diabetes (as assessed by measurement of glycosylated hemoglobin), the presence or titer of anti-insulin antibodies, the duration of the diabetes, the age of onset, the presence of complications, or the current insulin dose. These results indicate that varying amounts of insulin resistance may be present in Type I diabetes and cannot necessarily be explained by poor control or the presence of insulin antibodies.