We examined the spleens from four patients with childhood ITP. Numerous foamy cells were investigated in two cases, moderate number in one case, and a few in one case. PAP method using anti-human platelet antibody demonstrated the platelet antigen in the cytoplasms of these foamy cells, which were granular or reticular. Electron microscopically, many platelets in various stages of intracellular digestion from intact-appearing forms to myelin-like materials, were disclosed. Enzyme cytochemical electron microscopy revealed localization of acid phosphatase activity around the degrading platelets and vacuolated inclusions, but rarely in the myelin-like materials. We suggest that the mechanism of formation of the foamy cells in ITP is as follows; macrophages phagocytize many platelets, exhaust their lysosomal enzyme, and can not digest the engulfed platelets completely. Thus the partially degraded platelets remain as myelin-like materials in the cytoplasm of macrophages, which have foamy appearance in light microscopy.