This study examined the plasma glucose level and immunoreactive insulin (IRI) level in the course of 75g oral glucose tolerance test (OGTT) during pregnancy in 27 obese women (pre-pregnant body mass index (BMI) > = 24kg/m2) and 53 non-obese women (BMI < 24kg/m2). In women with normal glucose tolerance, the fasting IRI level, IRI area under a 3-h curve during 75gOGTT (IRI-AUC) and the net increase in IRI areas above fasting values under a 3-h curve (delta IRI-AUC) in 15 obese women were higher than those in 47 non-obese women (p < 0.01, p < 0.05, p < 0.03), but there was no difference between the two groups in the fasting plasma glucose level, BS-AUC or delta BS-AUC. In women with abnormal glucose tolerance, the fasting IRI level in 12 obese women was higher than that in 6 non-obese women (p < 0.01), but there were no significant differences in the other factors. The fasting IRI level was correlated with the fasting plasma glucose level, BS-AUC, IRI-AUC and BMI (p < 0.002, p < 0.001, p < 0.0001, p < 0.0001). These results show that there is a higher insulin level and lower systemic sensitivity to insulin in obese women than in non-obese women in all groups.