OBJECTIVE: Expression of intercellular adhesion molecule-1 (ICAM-1), a marker of endothelial dysfunction leading to damaging vascular disorders, in umbilical and placental vascular tissue of gestational pregnancies was compared to non-diabetic controls. METHODS: We included 32 pregnant women with gestational diabetes mellitus (GDM) and 28 women with normal ongoing pregnancies were taken as the control group. Pregnant women with GDM were selected from the ones who had glycosylated haemoglobin (HbA(1c)) values lower from 6%. CD54/ICAM-1 expression profile was evaluated by immunohistochemistry, and cellular localization was determined under light microscopy. The immunoreactivity was assessed using a four-tiered scale: 0-5% (0), 6-20% (+1), 21-50% (+2), 51-100% (+3). RESULTS: In gestational diabetic patient's umbilical artery, +1 immunostaining group was observed (62.5%), and in their placenta, the highest percentage was seen in the 0 immunostaining group (43.8%). Diabetic patient's umbilical vein has the highest percentage in the +1 immunostaining group. In the control group, in both umbilical artery and vein, the highest percentage was seen in the +2 immunostaining group (46.4%) and their placenta has the +3 immunostaining group with the highest percentage (57.1%). CONCLUSION: The main outcome of our study was that, although underlying diabetes does have some effects on the pregnant mother, fears of endothelial dysfunction leading to damaging vascular disorders are probably unfounded in well-controlled GDM women.