Anti-phospholipid antibodies are associated with first trimester abortions and late intra-uterine fetal death. The histopathology of 47 placentae from 45 women with intra-uterine fetal death, including 16 patients with anti-phospholipid antibodies, was studied in order to detect potential differences between placentae from women with and without these antibodies. Thirteen patients had systemic lupus erythematosus or lupus-like disease, including 6 women with anti-phospholipid antibodies. In placentae from patients with anti-phospholipid antibodies, a decrease in vasculo-syncytial membranes, fibrosis mainly in infarcted areas, hypovascular villi and thrombosis or infarction was seen significantly more often than in placentae from women without these antibodies. Of 17 placentae from 16 patients with anti-phospholipid antibodies, only 3 did not demonstrate signs of thrombosis or infarction. Thrombosis/infarction was significantly associated with a decrease in vasculo-syncytial membranes, fibrosis, hypovascular villi and an increase in syncytial knots. These findings are most likely to be the result of prolonged hypoxia due to thrombosis or infarction. It is concluded that thrombosis or infarctions are prominent features in placenta from patients with anti-phospholipid antibodies and intra-uterine fetal death. Consequently, antithrombotic treatment during pregnancy forms a rational approach in these patients.