Thrombotic thrombocytopenic purpura/Hemolytic uremic syndrome (TTP/HUS) is generally regarded to be a rare disease. The present study was undertaken to identify presenting features, prognostic variables, pathological features and outcome associated with TTP/HUS. The present study is a retrospective chart review of 68 patients treated with plasmapheresis for TTP/HUS at a single tertiary referral medical institution from 1980-1992. The annual number of patients with TTP/HUS treated with plasmapheresis increased from an average of one case per year in 1980 to nine cases per a year in 1992. The in-hospital mortality for patients presenting with TTP/HUS was 25%. Forty four percent of patients presented with an elevated serum creatinine, and 16% required hemodialysis support. Of the seven patients who survived and required hemodialysis support only two patients continued on dialysis. None of the patients presenting with a normal serum creatinine required dialysis at any time in their course. Patient age, sex, presenting platelet count, white blood cell count, hemoglobin level and presence of neurological disease were not significantly associated with death or need for dialysis. The histopathological features of TTP/HUS (fibrin/platelet thrombi in renal vessels and glomeruli, fibrinoid necrosis of vessel walls) were found in all five cases autopsied. The incidence of TTP/HUS may be increasing. Alternative possibilities for the increased frequency of cases seen include greater diagnostic suspicion and referral bias. Despite the use of plasmapheresis, mortality during the initial hospital admission was almost 25%. In retrospect prognosis could not be predicted based on admission biochemical or clinical variables. The majority of patients who developed acute renal failure and survived to hospital discharge recovered renal function and became independent of dialysis.