The Glycemic Index (GI) is a measure of the extent of the change in blood glucose content (glycemic response) following consumption of digestible carbohydrate, relative to a standard such as glucose. We have explored whether the reported GIs of foods are a sufficient guide to a person wishing to avoid large glycemic responses and thereby avoid hyperglycemia. For this purpose, volunteers carried out multiple tests of four foods, following overnight fasting, measuring the glycemic response over 2 H. The areas under the blood glucose/time curves (AUCs) were compared. Each food tester displayed individual, characteristic glycemic responses to each food, unrelated to any other tester's response. Wide variations (up to 5-fold) were seen between the average AUCs for the same test by different testers. The absolute magnitudes of the glycemic responses are important for individuals trying to control blood sugar and/or body weight, but using published GI lists as a guide to control the glycemic response is not fully informative. This is because in calculating the GI, individual glycemic responses to glucose are normalized to 100. GI values are, therefore, relative and are not necessarily a reliable guide to the person's actual individual AUC when consuming a food. Without knowledge of the person's characteristic blood glucose responses, reliance only on the GI may be misleading. © 2010 IUBMB IUBMB Life, 62(8): 637–641, 2010.