The intima of the elastic arteries undergoes progressive thickening with aging under hemodynamic stress. The earliest changes in humans have been recorded soon after birth, but rare intrauterine fetal cases are on record. We studied a case of intimal thickening that occurred during fetal life in the aorta of the hypervolemic partner of a pair of twins afflicted with placental transfusion syndrome. These in utero changes suggest that intimal thickening of elastic arteries may also occur any time during fetal life if the walls of these vessels are exposed to factors known to cause endothelial damage.