The ultrastructure of the arterial supply of the human placenta was studied in 15 pregnancies with severe fetal growth retardation. There were five patients with essential hypertension and superimposed pre-eclampsia, five patients with pre-eclampsia, and five patients with no hypertension. The patients were delivered by elective Caesarean section and biopsies of the placental bed were taken under direct vision. Extensive placental infarction was invariably present and the degree of infarction reflected the extent of pathological changes in the spiral arteries. In both hypertensive and normotensive patients the spiral arteries and basal arteries of the decidua showed occlusive atheromatous lesions with considerable fibrin deposition and accumulation of lipid-laden cells in both the intima and media of the vessel walls.