Blood glucose levels were continuously monitored in 70 subjects during a 75 g oral glucose tolerance test (OGTT). Subjects were divided into normal (N = 15), borderline (N = 31), diabetes with fasting glucose levels below 140 mg/dl (DM1) (N = 15) and diabetes with levels above 140 mg/dl (DM2) (N = 9). Three patterns of blood glucose curves were observed in each subject group; domed, biphasic and upward. The frequency of blood glucose patterns in each class of glucose tolerance group was as follows: in the normal group; domed 33.3%, biphasic 66.7%; in the borderline group; domed 67.7%, biphasic 29.0%, upward 3.2%; in the DM1 group; domed 66.7%, biphasic 13.3%, upward 20.0%; in the DM2 group; domed 77.8%, upward 22.2%. The frequency of patients with a biphasic pattern was significantly higher in the normal group than in the other groups. In the borderline group, almost all patients with a biphasic pattern were young or middle aged (< 60 years old). When the patients with fasting glucose levels below 140 mg/dl were analyzed, the mean peak time and peak value of blood glucose levels were significantly higher in patients with domed patterns than those with biphasic patterns. Indices of early insulin response to glucose load were significantly lower in patients with domed patterns than in those with a biphasic pattern. In conclusion, the pattern of the glucose curve in an OGTT is mainly dependent on the patient's early insulin response. Glucose intolerance with aging resembles diabetes from the standpoint of the pattern of glucose tolerance curves.