Plasma insulin responses and insulin resistance were determined in 75 subjects, defined as having a normal, borderline abnormal, or abnormal oral glucose tolerance test (OGTT). Although considerable heterogeneity of insulin response existed, most patients with abnormal OGTT's had insulin responses greater than normal; none had insulin responses less than normal. The degree of insulin resistance also varied, but most patients with abnormal OGTT's were also abnormally insulin resistant A significant correlation (r=0.64, p+/-0.001) existed between insulin response and the degree of insulin resistance. However, when both variables were taken into consideration, the entire population could be divided into two groups. One group was characterized by both normal insulin responsiveness and sensitivity, the other by increased insulin response, associated with greater insulin resistance. Most patients with abnormal OGTT's fell into the latter group, but some had glucose intolerance without either an exaggerated insulin response or insulin resistance. These results suggest that true heterogeneity exists in patients with abnormal OGTT's.