The present study was aimed at investigating the relationship between diabetes and coronary heart disease. 100 g oral glucose tolerance test was performed on 32 well-defined male non-obese patients with coronary heart disease (myocardial infarction and angina pectoris, 16 patients each; average age 47.5 years) and 12 non-obese controls (average age 46.2 years). The results were as follows: 1) Glucose tolerance was impaired in 19 of 32 patients, 2) Insulin response to glucose in patients with normal glucose tolerance was similar to that in controls, while insulin response in patients with impaired glucose tolerance showed a delayed peak. 3) The insulinogenic index of patients with normal glucose tolerance was the same as that of controls, while the insulinogenic index of patients with impaired glucose tolerance was lower, and 4) overnight fast serum cholesterol and triglycerides in patients with normal glucose tolerance showed similar levels to those in controls, but were elevated in patients with impaired glucose tolerance. These results suggest that impaired glucose tolerance observed in patients with coronary heart disease may result in part from “latent diabetes.