Castleman's disease is a giant lymph node hyperplasia which is developed at the mediastinum. The lesions are rare at the neck. Recently, we experienced on case of Castleman's disease of the neck, which was histopathologically diagnosed as a hyaline-vascular type of the Castleman's disease. To identify the composed cells, we performed and immunohistochemical study of Castleman's disease and a case of reactive hyperplasia of the lymph node using the MT1 as a T-cell marker and MB2 as a B-cell marker. In H-E stain, the Castleman's disease was different from the reactive hyperplasia of tonsil as the follows : proliferation of follicle without the difference between the medullar and the cortex of lymph node, proliferation of hyalinized capillaries, the extensive capillaries proliferation of which the endothelial cells were hypertropic between capillaries and a tight concentric layering of lymphocytes at the periphery of the follicles resulting in an onion-skin appearance. In immunohistochemical studies, the positive cell of MT1 represented the high density in the interfollicular area and the positive cell of MB2 represented the high density in the follicles in the Castleman's disease. The reactive hyperplasia of tonsil represented the high density of positive cell of MT1 in the interfollicular area but the positive cell of MB2 represented the high density in the follicule and interfollicular area. There was a difference between the Castleman's disease and the reactive hyperplasia of tonsil in the H-E staining and the immunohistochemical staining.