Interpretation of the results of an oral glucose tolerance test is simplified when the results are expressed as the Glucose Tolerance Sum for two or for three hours (GTS0-2 hr and GTS0-3 hr.). The GTS0-2 hr value is the sum of the 0, ½, 1 and 2-hr, glucose levels in venous blood when 1.75 gm. of glucose are administered per kilogram of body weight. Data from more than 1,100 such tolerances indicate that when GTS0-2 hr is 500 or less there is a 98 to 100 per cent probability that the test is clearly normal by the standards of the World Health Organization, British Diabetic Association, United States Public Health Service, and Fajans and Conn of what is and what is not diabetes mellitus. On the other hand, when the GTS0-2 hr is 801 or higher the test would meet each of the four sets of above cited criteria for diabetes. Since those values between 501 and 800 are variably classifiable as normal or abnormal by these same four standards, this represents the equivocal zone. The GTS0-3 hr value is based on the sum of 0, 1, 2, and 3-hr, blood glucose levels in the above tolerance tests. Using the same criteria, clearly nondiabetic tolerance tests yield GTS0-3 hr values of 450 or less, while those deemed to be diabetic have sums of 701 or higher. Intervening GTS0-3 hr values of 451 to 700 represent the equivocal zone. Tolerance tests with GTS values in the equivocal zone should be repeated at intervals of months or years. In our experience equivocal tests generally move toward or into the abnormal range, a minority move back into the normal zone, and some remain equivocal.