Estimated fraction of the various forms of secondary ITP based on clinical experience of the authors. The incidence of HP ranges from approximately 1% in the United States to 60% in Italy and Japan. The incidence of HIV and hepatitis C approximates 20% in some populations. Miscellaneous causes of immune thrombocytopenia, for example, posttransfusion purpura, myelodysplasia, drugs that lead to the production of autoantibodies, and other conditions, are not discussed further in this review. ALPS indicates autoimmune lymphoproliferative syndrome; posttx, post–bone marrow or solid organ transplantation; SLE, systemic lupus erythematosus; APS, antiphospholipid syndrome; CLL, chronic lymphocytic leukemia; CVID, common variable immune deficiency. In the absence of a systematic analysis of the incidence of secondary ITP, the data shown represent our combined assessment based on experience and reading of the literature. Professional illustration by Paulette Dennis.