The aim of this study was to measure oxidant/antioxidant status in maternal and cord plasma and in placental tissue in gestational diabetes and to correlate the results with the quality of glycemic control of the mother. To achieve this, blood and placental tissue samples have been obtained from pregnant women with gestational diabetes mellitus (GDM) and from the umbilical cord of their fetuses. The same samples have been collected from pregnant women without GDM. In all the samples, oxidant and antioxidant parameters have been studied. It has been observed that the antioxidant defense system was impaired; xanthine oxidase, which is the main free radical-producing enzyme (XO) in the living cells, was activated; and oxidation reactions were accelerated in the samples obtained from patients with GDM. Results suggest presence of oxidant stress in the gestational diabetes, the reason probably being impaired antioxidant defense mechanism and increased free radical production through XO activation.