Investigators with the RADAR (Research on Adverse Drug Events and Reports) project identified cases of ticlopidine- and clopidogrel-associated TTP with the use of pharmacovigilance methods that have been described previously (3- 5,13- 14). Thienopyridine-associated TTP cases were identified from 4 sources: 1) voluntary reports submitted to MedWatch, the Food and Drug Administration’s Safety Information and Adverse Event Reporting System (n = 29); 2) published case series or reports from MEDLINE/PubMED, using MeSH terms ticlopidine or clopidogrel, thrombotic microangiopathy, and TTP (n = 40) (4- 5,15- 16); 3) direct queries of hematologists and apheresis directors in 8 large apheresis centers in geographically dispersed metropolitan areas (Charles Bennett, MD, PhD, Chicago, Illinois; Joseph Kiss, MD, Pittsburgh, Pennsylvania; Thomas Ortel MD, PhD, and Nicholas Bandarenko, MD, Raleigh-Durham, North Carolina; Josh Levy, MD, and Nurit Begani, RN, Los Angeles, California; William Bell, MD, PhD, Baltimore, Maryland; Leo J McCarthy, MD, Indianapolis, Indiana; Jean Connors, MD, Boston, Massachusetts; and Joel Moake, MD, Houston, Texas; n = 42); and 4) a national referral laboratory in Japan (Yoshihiro Fujimura; n = 17). A validated case report form was used to collect data on sociodemographic characteristics, thienopyridine use, clinical data—platelet count (per mm3), hemoglobin level (g/dl), serum creatinine (mg/dl), neurologic findings (altered mental status, seizure, stroke, or coma)—use of TPE, and survival (4- 5). Inclusion criteria were thienopyridine use before the development of thrombocytopenia (platelets <50,000/mm3) and microangiopathic hemolytic anemia on peripheral blood smear, without the presence of any other identifiable cause, such as disseminated intravascular coagulation, cancer, or preeclampsia. Those cases that did not fulfill or report all of the required inclusion criteria were excluded from analysis.