There have been limited studies of large numbers of patients (pts) with ITP other than those describing platelet antibodies (AP). Contemporaneous data on the prevalence of| multiple AI markers and aTPO in ITP have not been reported. We are conducting a prospective cross-sectional study of pts 4yrs of age previously diagnosed with FTP. We excluded pts with HIV, hepatitis and other conditions causing thrombocytopenia. Pts are characterized with respect to demographics, platelet count (PLT), and the presence of AI; markers and aTPO. The study was started in March, 2000 at 11 sites in North America. We report here the results from the first 67 pts, 61 adults [A] and 6 pédiatrie [P] (200 targeted). Median (m) age was 42 yrs (range 9 to 90), and gender was 43(64%) females and 24(36%) males. In A the m age at diagnosis was 9 yrs older for males (46) compared to females (37). Splenectomy was reported in 30(45%) pts (29A, IP). The m PLT for all ptsi was 53xlO'/L, (A 57xlO'/L; P 34xlO/L). All but 2 pts had reported bleeding episodes and 9 pts reported 1 or more prior thrombotic event. Twenty-six (39%) pts tested positive (+) for one or more of the following: ANA (8A.3P), rheumatoid factor (3A.1P), and direct(d) Coombs (13A.1P). AP antibody status was reported for 38 pts, 27 were + at the time of testing (25β6A, 1/2P). Pts with +d Coombs or +AP antibody have lower m PLT (34 vs 61xlO'/L + vs -; 40 vs 91xlO'/L + vs -, respectively). Pts sera (57) were tested for aTPO in several assays. Five were reactive in the screening assay. This reactivity was non-specific, therefore, all pts tested thus far were negative for aTPO. We conclude that positive AI markers and thrombotic events commonly occur in ITP, however, aTPO, appear to be rare. This profile of tests may allow the identification of clinically distinct sub-populations of pts with ITP with specific management needs.