The characteristics of the glucose and insulin responses during the glucose tolerance test (GTT) in obese people as a group have not been established. We analyzed glucose and insulin levels during GTT in 160 healthy obese patients who averaged 42% over ideal body weight. Statistical upper limit of normal for 2-h glucose was 260 mg/dl in women and 206 mg/dl in men. Although there was a significant correlation between insulin and glucose levels in both sexes and between insulin and degree of obesity in women, r values were relatively low (r less than 0.4 for all). High insulin levels and delayed peak insulin were present in the majority of patients with normal GTT and absent in many of the most obese patients. Results indicate that upper limits of normal glucose for GTT in the obese are much higher than currently accepted criteria.