BACKGROUND: Thrombotic thrombocytopenic purpura-hemolytic uremic syndrome (TTP-HUS) is a diagnosis of exclusion when a patient presents with the sine qua non findings of thrombocytopenia and microangiopathic hemolytic anemia without an identifiable cause. Although most patients respond to therapeutic plasma exchange (TPE), a significant number of patients relapse. The aim was to determine if clinical, laboratory, and/or treatment features could predict response and/or relapse. STUDY DESIGN AND METHODS: This study was a retrospective review of adults with TTP-HUS treated with TPE at our institution from January 1996 to February 2004. RESULTS: The study population consisted of 90 patients (69% female) with mean age of 45 years and mostly obese (65%). The majority of cases were considered idiopathic. Ten patients died (11%) from the disease before achieving a response, whereas 79 percent were considered responders. Obesity and severe anemia at presentation were predictors of response to TPE (p = 0.0126 and p = 0.0071, respectively). Among the responders, 28 percent relapsed in a median of 14 months. Male sex, severe thrombocytopenia (mean ± SD, 13 × 109 ± 8 × 10 9/L), and higher lactate dehydrogenase pre-/posttreatment ratio were associated with relapse (p values of 0.0141, 0.0199, and 0.0407, respectively). ADAMTS-13 values were not obtained on enough number of patients to provide important data. CONCLUSION: Although patient and laboratory characteristics associated with response and relapse were identified, there was significant overlap between patient groups. Thus, our findings offer preliminary evidence and do not yet justify short- or long-term changes in the management of patients with TTP-HUS. © 2007 American Association of Blood Banks.