This study compared the results of a 75-g, 3-hour oral glucose tolerance test with those of a traditional 100-g oral glucose tolerance test. Thirty-two pregnant women participated in the study. Each patient served as her own control, undergoing both a 100- and a 75-g oral glucose tolerance test within 1 week. Despite a strong positive correlation between the results of the two tests, the 1-, 2-, and 3-hour glucose values of the 100-g glucose load were significantly higher than the comparable values of the 75-g oral glucose tolerance test. Sixteen of 32 women were diagnosed as having gestational diabetes mellitus using the National Diabetes Data Group criteria and the 100-g oral glucose tolerance test, whereas only 6 of these 16 women would have been identified with the 75-g oral glucose tolerance test. If data from one test are to be compared with the other, new thresholds of glucose abnormality need to be developed.