Three cases of postpartum hemolytic uremic syndrome (HUS) are presented. Symptoms of acute renal failure, hypertension and microangiopathic hemolytic anemia with thrombocytopenia occurred 10, 17 and 24 days after delivery. Despite early heparin therapy in all cases, one patient went into terminal renal failure needing chronic hemodialysis, with persistent hypertension which became uncontrollable requiring bilateral nephrectomy 6 months later. The second patient had diuresis one month after starting hemodialysis, but 3 months later developed malignant hypertension. Slight improvement in renal function with persistent hypertension occurred after hemodialysis for 20 months. The third patient showed complete clinical recovery after 2 months. Pathological examination of renal tissue showed the typical lesions of thrombotic microangiopathy (TMA). However, striking differences were observed in the lesion seen in early and late specimens. Early lesions could be differenciated from infancy TMA because the medium-dize arteries were more severely involved. Late lesions were variable, ranging from minor changes in glomeruli and blood vessels, via ischemic and sclerotic lesions in glomeruli with arteriolosclerosis, to the vascular and glomerular lesions seen in malignant nephrosclerosis. There was a good correlation between the renal pathology and the clinical outcome of the patients. HUS with renal TMA as a cuase of postpartum renal failure has been reported in 49 patients with a fatal outcome in 61%. The pathogenesis of the syndrome probably involves a primary endothelial damage. This causes local renal intravascular coagulation in the presence of the usual postpartum hypercoagulable state. This is shown by the presence of fibrin-fibrinogen in glomeruli and vessels, increased plasma fibrin degradation products, thrombocytopenia and lowered levels of coagulation factors. There is little hematological or pathological evidence fo disseminated intravascular coagulation or an immune-complex disease. Hypocomplementemia seen frequently is probably due to local C3 activation via the alternative pathway.