An association between HIV infection and Thrombotic Thrombocytopenic Purpura (TTP) remains controversial.We conducted a cross-sectional analysis of 39 consecutive, non-referral cases of TTP encountered over 9 years ata single institution. Thirteen cases had HIV infection. The patients were treated with daily plasma exchange untilremission. The mean (standard deviation) duration of follow-up was 48 (37) months. Seven patients died. TTPcaused 3 of the 4 deaths in the HIV- group but none of the 3 deaths in the HIV+ groups. The age and sex adjustedincidence rate of TTP was 14.5 cases per 106 person-years. The relative risk of TTP was 38.5 (95% confidenceinterval, 19.7-75.0) for HIV infection, 2.7 (1.3-5.7) for female gender and was not increased for the black race.Neither HIV infection nor gender affect the overall and relapse free survivals. While relapse continued to occurthroughout the follow-up period in the HIV- group, it did not occur in the HIV+ group after the first year. We concludethat HIV infection is a major risk factor of TTP. The risk of late TTP relapse is low in HIV infected patients treatedwith anti-retroviral therapies.