Placentae from controls and two groups of diabetic women (one White classes A, B, C and the other classes D, F/R) were collected at 37-42 weeks of gestation. Tissue sections were analysed using stereological methods in order to quantify the growth and maturational status of villi. Birth and placental weights were recorded and placentae sampled in a systematic manner. Fields of view on formalin-fixed, paraffin-embedded sections were analysed to obtain estimates of volumes, surface areas, lengths and diffusion (harmonic mean) distances. Comparisons were drawn using three-way analyses of variance with group, mode of delivery and sex of newborn as the principal effects. Mean weights were similar in controls and diabetic groups. Diabetic placentae had a more voluminous fetal capillary bed of greater length, diameter and surface area. In addition, the diffusion distances across fetal plasma (erythrocyte to endothelium) were shorter. Stromal diffusion distance and villous diameter were greater in vaginal deliveries. Interaction effects influenced also villous capillarization, capillary volume, capillary diameter, trophoblast thickness and stromal thickness. Our results emphasize the importance of adaptations on the fetal side of the diabetic placenta. They show that changes can affect the placentae of appropriate-for-age as well as large-for-age babies and provide no evidence that they increase with the severity and duration of diabetes.