BACKGROUND: Although 75 g-OGTT at PPT 6 week is necessary to diagnose persistent PPT glucose intolerance (PPGI) in GDM patients, it 1s difficult to perform this test because many patients drop-out during the follow-up period. Thus we tested whether OGTT done at PPT 1 week can substitute OGTT at PPT 6 week in diagnosing PPGI in GDM patients. METHOD: In 370 GDM patients, 75 g-OGTT was performed at PPT 1 week and repeat OGTT was done in 196 patients at PPT 6 week. Results of OGTT were classified as normal glucose tolerance(NGT), impaired glucose tolerance(IGT), and diabetes mellitus (DM) according to National Diabetes Data Group(NDDG) criteria. Changes in glucose tolerance state between PPT 1 and 6 week were assessed, and the predictability of clinical characteristics for these changes were investigated by logistic regression analysis. RESULTS: Among 370 GDM patients who performed OGTT at PPT 1 week, 79.4% had NGT, 12.2% had IGT, and 8.4% had DM. 53% (196/370) of subjects repeated OGTT at PPT 6 week. In OGTT at PPT 6 week, 77.6% (152/196, 140/149 in NGT, 4/26 in IGT and 8/21 in DM) were in the same glucose tolerance state as at PPT 1 week. The glucose tolerance improved in 14.8% (29/196, 16/26 in IGT and 13/21 in DM) and deteriorated in 7,6% (15/196, 9/149 in NGT and 6/26 in IGT). 94%(140/149) of patients who had NGT at PPT 1 week had NGT at PPT b week and 48.9/o (23/47) of patients who had abnormal glucose tolerance at PPT 1 week had abnormal glucose tolerance at PPT 6 week. Mean fasting plasma glucose level on OGTT became lower at PPT 1 week than during pregnancy (4.6+/-0,8 vs 5.1+/-1.2mmol/L, p<0.05) and became higher at PPT 6 week than at PPT 1 week (5.4+/-1.1 vs 4.6+/-0.8 mmol/L, p<0.05). Mean plasma glucose level at 2 hour after glucose load was significantly lower at PPT 6 week than at PPT 1 week (7.2+/-2.7 vs 8.3+/-2.5 mmol/L). When the subjects were grouped into NGT, IGT, and DM according to glucose tolerance state at PPT 6 week, the NGT group already showed normal glucose tolerance at PPT 1 week. The IGT and DM group showed slightly lower glucose levels at PPT 1 week than during pregnancy but became high to the level during pregnancy at PPT 6 week. In the patients group showing deterioration in glucose tolerance state between PPT 1 and 6 week, prevalence of insulin treatment was higher (63.4% vs 9.4, 20.7%), the gestational age at diagnosis of GDM were lower (25.0+/-6.2 week vs 29.8+/-3.3, 29.9+/-4,8 waek), and prepregnant weight was higher (113.4+/-21.2% vs 102.5+/-12.4, 102.4+/-14.6%) than those in the patients groups showing no change and improvement in glucose tolerance state, Weight gain until diagnosis of GDM during pregnancy(5.7+/-4.4kg vs 9.4+/-3.4kg) and weight change between prepregnancy and PPT 5 week(-1,3+/-3.5kg vs 1.5+/-29kg) was smaller in the deterioration group than those in the no change group. Logistic regression analysis performed using improvement and deterioration of glucose tolerance state between PPT 1 and 6 week as an outcome of interest revealed that an earlier diagnosis of GDM and a smaller weight at PPT 6 week than prepregnant weight were independent predictors for deterioration of glucose tolerance between PPT 1 and 6 week. In conclusion, OGTT done at PPT 1 week can substitute OGTT at PPT 6 week in a large subgroup of GDM patients who has NGT at PPT 1 week without any risk factors for deterioration in glucose tolerance.