The pathology of B-lymphocytes in the bone marrow of patients with idiopathic thrombocytopenic purpura (ITP) has not been well described, even though B-lymphocytes may be involved in the etiology of ITP. We retrospectively reviewed the medical records of 73 ITP patients between January 1997 and June 2005 with platelet counts of < 50 x 10⁹/L. Bone marrow clots were available for pathological review in 56 patients who were classified into 3 groups based on the results of the bone marrow clot examination : Group A (21 patients) had increased CD20⁺ lymphocytes (≧1% of nucleated cells) and megakaryocytes with morphologic changes ; Group B (21 patients) had morphologic changes but no increase in CD20⁺ lymphocytes ; and Group C (14 patients) had neither morphologic changes nor increased CD20⁺ lymphocytes. Multivariate analysis showed that, compared to Group A, Group B had a significant prognostic factor (p = 0.04 ; odds ratio, 6.65 ; 95% confidence interval, 1.09 to 40.54) for achieving complete response, while Group C had a significant prognostic factor for any treatment response (p = 0.04 ; odds ratio, 14.26 ; 95% confidence interval, 1.08 to 188.02). Thus, ITP patients can be classified with different clinical outcomes based on immunohistopathological examination of bone marrow clots. [J Clin Exp Hematopathol 48(1) : 11-15, 2008]