The mean platelet volume (MPV) is an indicator of the average size and activity of platelets. Elevated MPV values are associated with larger and more active platelets and perceived as a new independent cardiovascular risk factor. The aim of this study was to determine the MPV in women with gestational diabetes mellitus (GDM) and to determine the correlation of MPV with metabolic parameters in GDM. We retrospectively analyzed 30 women with GDM and 38 body mass index-matched women with healthy pregnancies as controls. MPV and platelet counts were recorded in the third trimester and at postpartum 6-12 months for GDM group and in the third trimester for control group. Third-trimester MPV was significantly higher in GDM group compared to control group (8.8 +/- 1.0 versus 8.1 +/- 0.7 fl, p = 0.002). In women with GDM, there was a significant decrease in MPV in the postpartum period (8.8 +/- 1.0 versus 8.1 +/- 0.8 fl, p<0.001). Fasting plasma glucose levels and glucose area under the curve were positively correlated with third trimester MPV (r = 0.346, p = 0.007 and r = 0.346, p = 0.02, respectively). Our results indicate that MPV is increased in GDM. Monitoring MPV, which is widely available in clinical practice, may potentially identify women who will develop gestational diabetes during pregnancy.