The thrombotic thrombocytopenic purpura/ hemolytic uremic syndrome (TTP/HUS) is a rare disorder characterized by microangiopathic hemolysis and thrombocytopenia. We have undertaken a retrospective analysis of the clinical characteristics, treatment outcome, and prognosis of 48 patients diagnosed and treated in our institution during a 13-year period. Among our patients 22 (46%) had fever, 35 (73%) neurological abnormalities, and 22 (46%) renal impairment at presentation of the syndrome. All patients were treated with a multimodality regimen including plasma exchange, steroids, antiplatelet agents, and IgG infusion. Of the 48 patients, 41 achieved complete remission, two had a partial response, and five had no response and died of progressive disease. Within a median follow-up period of 40 months, ten of the 41 patients who had achieved remission relapsed, most of them within the first 2 years, and nine of these responded promptly to plasma exchange therapy. Eight deaths were observed, seven of refractory disease and one in fourth relapse. The analysis of prognostic factors revealed advanced age and severe renal impairment (creatinine levels above 2 mg%) as the only parameters associated with treatment failure and poor outcome. However, none of the pretreatment characteristics proved to be of prognostic value regarding the probability of relapse. In conclusion, TTP/HUS represent a syndrome of variable clinical expression and aggressiveness. The use of a multimodality regimen in our series produced a high response rate. Nevertheless, the early identification, based on clinical characteristics, of poor-prognosis cases that probably need more or alternative forms of treatment is an issue that remains to be elucidated in prospective trials.