Maternal glucose tolerance in pregnancy is determined by an oral glucose tolerance test (oGTT). The presented study deals with the question whether the risk of the fetus developing hyperinsulinism can be correctly estimated by the result of the oGTT. Investigations were made if there is a correlation between the oGTT and the mean blood glucose levels (MBG) and the fetal glucose metabolism measured by amniotic fluid insulin (AFI) at birth. 158 amniotic fluid samples were collected during labour. In 136 samples insulin levels below the threshold of 7 microU/ml were found, in 22 samples above 7 microU/ml. An oGTT was performed in all pregnancies (threshold: 95/165/145/125 mg%). 52 women showed normal oGTT, 28 had impaired glucose tolerance (IGT) with one pathologic value and 78 women had gestational diabetes (GDM) with two elevated values. Elevated insulin levels > 7 microU/ml were found in 6% of the cases with normal oGTT, in 29% of the cases with IGT and in 14% when GDM was diagnosed (p = 0.02). The MBG was significantly higher in cases with elevated AFI than with normal AFI, 92 mg% versus 83 mg% (p = 0.02). Therefore hyperinsulinism of the fetus was found twice as often in cases with IGT than in GDM diagnosed by oGTT. Borderline glucose tolerance with only one pathologic value in the oGTT has more affect on the fetal glucose metabolism than has been assumed up to now. This may be caused by insufficient therapeutic intervention and deterioration of glucose tolerance during pregnancy. The diagnosis of IGT should be followed by therapeutic efforts and intensive care of the fetus as in cases of GDM.