A unique vascular lesion has been identified in 32 placentas sent to the Michigan Placental Tissue Registry, East Lansing, Mich. Half of the associated infants are stillborn; 11 of 16 liveborn were in distress or small for gestational age. Placental vessels show thrombosis, endothelial, and medial hyperplasia and narrowing or obliteration of the lumen. A microangiopathic process is suggested by intraluminal fragmentation of erythrocytes with diapedesis of intact and fragmented RBCs through vessel walls. Red blood cell fragments and hemosiderin are present within villous stroma. Nucleated erythrocytes in placental vessels suggest fetal hypoxia. On gross examination, the placentas are meconium stained. The umbilical cords are frequently edematous, redundant, and around a fetal part; gradual narrowing of placental vessels may be related. Chronic villitis is present in 75% of the cases, intranuclear inclusions have been identified in 10%. An infectious agent, possibly viral, is suggested; toxic and chemical substances must be considered.