The morphology of the placental bed spiral arteries was studied in 68 pregnancies complicated by fetal growth retardation and in 40 pregnancies with a normally grown fetus. When the birth weight was normal the extent and depth of physiological vascular changes were normal except in those pregnancies complicated by pre-eclampsia. When the birth weight was low and the mothers were normotensive the extent and depth of physiological vascular changes were either normal or restricted, and in all patients with hypertension and a baby with low birth weight the physiological changes were restricted to the decidual segments of the spiral (uteroplacental) arteries. Acute atherosis was only found in pregnancies complicated by hypertension, particularly if there was proteinuria. We do not believe that there exists an arteriopathy which is common to hypertensive and normotensive pregnancies complicated by fetal growth retardation.