Two hundred and eighteen cases of hemorrhagic endovasculitis (HEV), a recently recognized abnormality of human placentas, were identified from placentas submitted to the Michigan Placental Tissue Registry over a 2.5-year period. HEV appears to focus on fetal placental blood vessels with resultant fragmentation and destruction of fetal RBCs, hemorrhage into villous stroma, microthrombi in villous capillaries and non-exudative necrosis of medium sized chorionic vessels. Placentas without HEV submitted for evaluation from the same hospitals were selected for comparison. Women with HEV placentas were of similar age, race and parity as control women. The sex of the fetus of HEV cases was somewhat more often female (57% vs. 48%, P = 0.05). The presence of HEV appeared to have a deleterious effect on the outcome of pregnancy: 52% (112/218) of the HEV placentas were associated with stillborn infants, in contrast to only 22% (89/400) of control placentas. The difference in the proportion of stillbirths was greatest when the gestational age was 25 weeks or greater. Other significant positive associations with HEV included the presence of meconium staining, intrauterine growth retardation, smaller placentas, and maternal hypertension or toxemia. Pathologic findings associated with HEV were chronic villitis, erythroblastosis, thrombosis and cord abnormalities. No association was found with infant Apgar score or fetal anomalies. The association of HEV with a high proportion of stillbirths in the registry suggests that further understanding of this lesion might shed light on the problem of unexplained stillbirth.