Maternal vascular dysfunction may alter the feto-placental environment and be associated with aberrant placental vascular development. We therefore aimed to determine: (1) if pregnancy in the insulin dependent (type I) diabetic woman is associated with increased maternal endothelial dysfunction, and (2) if the ultrastructure of the terminal placental villus, the functional exchange unit, is altered in these pregnancies. As an index of maternal vascular function circulating concentrations of defined endothelial-derived cell adhesion molecules were assessed. An ELISA was used to quantify the concentrations of the cell adhesion molecules throughout diabetic and control pregnancies and in matched non-pregnant women. The circulating concentrations of the cell adhesion molecules: E-selectin and ICAM-1 were increased in non-pregnant diabetic subjects compared to non-pregnant controls. These cell adhesion molecules interact with neutrophils and provide evidence of endothelial dysfunction in our population of non-pregnant diabetic women. In contrast, during pregnancy there was no difference in the circulating concentrations of ICAM-1 between diabetic and control groups. The concentration of E-selectin was significantly reduced when measured in the pregnant compared to the non-pregnant diabetic cohorts. These findings suggest that pregnancy, in our population of diabetic women, may actually be a time of vascular well-being. We hypothesize that this is a reflection of the improved glycaemic control achieved by these women during pregnancy. The ultrastructure of the terminal placental villi from ten diabetic and control pregnancies was assessed by three techniques: transmission electron microscopic determination of cross-sectional architecture, scanning electron microscopy of specially prepared placental vascular casts and immunohistochemical assessment of villous stromal composition and cell turnover. The terminal villi of diabetic placentae were highly comparable to those of control placentae. Specifically we demonstrated no significant difference on comparison of villous diameter, villous capillary diameter, cytotrophoblast or syncytiotrophoblast nuclei number and turnover, or stromal matrix content.