This study was undertaken in order to evaluate the plasma glucose response to oral glucose tolerance test (OGTT, 100 g) and the HbA1c values in pregnant women at different gestational ages. One-hundred twenty-nine OGTTs have been performed in 75 pregnancies. The results obtained show a decrease in glucose tolerance during pregnancy. Mean HbA1c value was significantly higher in women with gestational diabetes mellitus, but values of subjects with gestational diabetes and normal glucose tolerance overlapped widely. In conclusion, HbA1c is not a sensitive parameter in the diagnosis of gestational diabetes mellitus. Further studies are necessary to evaluate its specificity and prognostic significance.