Thrombotic thrombocytopenic purpura and hemolytic uremic syndrome are severe life-threatening disseminated thrombotic microangiopathies (TMA). Although many cases are idiopathic, TMA can occur in association with pregnancy, malignancy, autoimmune diseases, and HIV infection. We retrospectively analyzed the cases of 17 patients with TMA coexistent with HIV infection admitted to our institution. Median T-cell count at presentation was 28 cells/mm3 Patients presented with severe thrombocytopenia (median platelet count 19 x 10(9)/L) and high lactate dehydrogenase levels (median 1,057 U/L). The majority of patients (82%) presented with renal dysfunction. Forty-one percent of patients had fever and 29% had neurological signs at presentation, which were associated with inferior outcome. Despite plasma exchange, inpatient mortality for the first TMA episode was 47%. Some patients relapsed following an initial TMA episode. However, there were responders with remissions lasting 5 years. We conclude that TMA, coexistent with an HIV-associated low CD4 count, is a treatable condition. Considering TMA as an AIDS-defining illness may help clinicians recognize this syndrome earlier, leading to prompter treatment and improved survival rates.