The thyroid glands of nine patients with Hashimoto's thyroiditis were studied by immunoperoxidase method for immunoglobulin (Ig) and thyroid hormone, and of these four tissue specimens were further examined by electron microscope. Immunoperoxidase method for Ig revealed that about 63% of the infiltrating cells contained Ig, and that about 90% of such Ig‐containing cells had IgG. IgG‐containing cells seemed to secrete autoantibodies. Immunoperoxidase method for thyroid hormone and electron microscopy revealed that there was a good correlation between the morphological features of the thyroid follicle and its immunohistochemical staining pattern. In follicles composed of columnar cells, the colloid and cytoplasm of some epithelial cells were immunohistochemically stained. Dense deposits, regarded as immune complexes, were observed in the basement membrane of these follicles, and lymphocytes were seen between the adjacent cells. When similar deposits appeared in the basement membrane of such follicles, 4 or more lymphocytes per follicle could be seen among the epithelial cells. The present findings seem to indicate that antibody‐dependent cell‐mediated cytotoxicity plays an important role in the pathogenesis of Hashimoto's thyr