The structure of the spleens of 12 patients with idiopathic thrombocytopenic purpura (ITP) who underwent splenectomy was studied. The white pulp was characterized by the presence of large numbers of lymphatic nodules containing highly reactive germinal centers. The marginal zone contained large numbers of vessels surrounded by plasma cells. These findings imply active antibody production and are consistent with the concept that the spleen is a major source of antiplatelet antibody production in ITP. Large numbers of platelets in various stages of degradation were seen in cords, particularly in the marginal zone. They appeared both extracellularly and with the cytoplasm of macrophages. It appeared that platelet destruction was initiated by focal cytoplasmic degradation and resulted in formation of cellular debris. These findings indicate that the spleen plays a fundamental role in the pathogenesis of ITP, involving synthesis of antiplatelet antibody, localization of platelets in a milieu rich in this antibody, and subsequently, phagocytosing of immune-damaged platelets.