Objective. Gestational diabetes mellitus (GDM) is defined as glucose intolerance that is detected for the first time during pregnancy. Normal pregnancy induces insulin resistance through the diabetogenic effects of placental hormones. Glucose tolerance test results in twin and singleton pregnancies were compared in this study. Subjects and Methods. A total of 360 pregnant women were Studied. 200 women (mean age 31.60 +/- 2.10 yr) had singleton pregnancies (Group I) and 160 women (mean age 28.20 2.70 yr) had twin pregnancies (Group II). 50- g, 1- hour glucose tolerance test was conducted oil the first prenatal visit. An abnormal glucose screen defined as glucose >= 140 mg/dL was followed by a 100g, 3-hour glucose tolerance test. Gestational diabetes was defined as the presence of two or more abnormal values during the 3-hour test. Results. Gestational diabetes was found in 4 of the 200 (2%) singleton pregnant women and 8 of the 160 (5%) twin pregnant women. Group I (Singleton) was further divided into two Subgroups according to whether the 1-hr plasma glucose level was 140 ma/dL (Group Ib). Likewise, Group II pregnancies was also divided into two subgroups oil the same basis. Mean screening test glucose levels were found to be 127.8 +/- 14.94 mg/dL ill Group la and 150.8 +/- 18.1 mg/dL in Group Ib women. Mean screening test glucose levels of Group IIa subjects was 92.80 +/- 18.30 mg/dL while that of Group IIb Subjects was 154.8 +/- 27.0 mg/dL. Mean 1(st) h glucose levels of 100-g glucose tolerance test was found to be 131.4 +/- 32.58 mg/dL in Group I, and 112.5 +/- 39.6 mg/dL in Group II. Mean 2(nd) h glucose tolerance test values were 133.2 +/- 28.8 mg/dL in Group I and 100.6 +/- 28.8 mg/dL in Group II. Mean 3(rd) h glucose tolerance test values were 107.6 +/- 23.58 mg/dl in Group I and 72 +/- 16.9 mg/dL in Group II. Conclusion: Glucose screening results and 100-g, 3- hour glucose tolerance test values have been found to be lower in twin pregnancies than in singleton pregnancies. Therefore, we suggest that these findings be taken into account in developing diagnostic criteria for gestational diabetes ill twin or more pregnancies.