Fifty-two cases of umbilical cord thrombosis from 3 patients populations are analyzed and compared with 68 cases from the literature. The incidence of cord thrombosis is approximately 1/1300 deliveries, 1/1000 perinatal autopsies, and 1/250 high-risk gestations. There is a slight male predominance. Umbilical vein thrombosis occurs more frequently than thrombosis of one or both umbilical arteries, but poor fetal outcome is more likely with arterial thrombosis. The mechanism of fetal death when only one umbilical artery is thrombosed is illustrated and discussed. The strong association between cord thrombosis and perinatal morbidity and mortality is not noted among prospective cases but, when present, is related to additional umbilical cord abnormalities, obstetrical complications, or systemic fetal conditions that are the likely cause of both the thrombosis and the poor fetal outcome. The pathogenetic relationship between cord thrombosis and these associated conditions is discussed, and it is concluded that cord thrombosis is a marker of both the severity of these conditions and the likelihood of poor fetal outcome.