Twenty-nine pregnant women with gestational diabetes mellitus (GDM) diagnosed before the 34th gestational week had three intravenous glucose tolerance tests (IVGTT) performed during pregnancy and a follow-up with OGTT post partum. The women with a normal OGTT post partum had a significant decrease in fasting serum glucose from the 33rd to the 38th week in pregnancy (4.8-->4.0 mmol/l, p < 0.05). However, the women with a diabetic/borderline OGTT showed no decrease in fasting serum glucose during the same period (5.1-->5.0 mmol/l). The K-value (the diminution rate of blood glucose) of the IVGTT in week 38 was significantly lower in women with puerperal diabetic/borderline OGTT compared with women with a normal post partum OGTT (1.05 +/- 0.07 vs. 1.32 +/- 0.08 -10(2) x mmol/l x min-1, respectively, p < 0.05). Diabetic or borderline diabetic OGTT in the first week post partum was significantly associated with a decrease in the K-value from week 33 to 38 (p < 0.05). Early diagnosis of GDM was found to be associated with a pathological OGTT post partum (p < 0.05). Five of 22 women (23%) with previous GDM had a diabetic and one (5%) a borderline OGTT at follow-up four to thirteen months post partum. High fasting serum glucose levels during the last trimester in GDM can identify the women at risk of diabetic/borderline OGTT post partum.