Although chronic thyroiditis has been recently noticed as an autoimmune disease, the histological classification based on clinico-pathological correlation is still vague at present. The specimens subjected to this study were taken by exploratory excision from 63 patients who were clinically diagnosed as chronic thyroiditis. Histological observation was done regarding the progress of pathologic changes in the thyroid gland. Lymphoid type (14 cases): lymph-follicle formation with scattered infiltration of lymphocytes was the predominant feature in this group. No definite alteration of the thyroi- epithelium was observed. Some of the so-called focal thyroiditis may be included in the early stage of this group. Some of them may develop into tremendous proliferation of lymphoid tissue in its late stage. Oxyphilic cell type (34 cases): oxyphilic alteration of the epithelium was predominant, but lymphocytic infiltrations observed in varying degrees was not an essential feature of this group. Fibrosis may possibly become more prominent with destruction of thyroid follicles in the late stage. Hyperplastic type (9 cases): most follicles were lined by cuboidal or tall epithelium with papillary infolding and contained thin colloid and some cell aggregates. This type of thyroiditis also may result in a proliferation of interlobular fibrous tissue in its more advanced stage. Fibrous type (6 cases): this type of thyroiditis is believed to be a late stage of the other three types. The original type of thyroiditis, however, was not clear because of advanced fibrosis. Evidences of the progress of the histological changes were observed in 12 out of 40 patients in whom the specimens were taken from both lobes and in two others whose specimens were taken twice at different stages. Clinical correlation of this classification was studied. The lymphoid type was found in 14 out of 53 female patients, but in none of 10 males. In age distribution the hyperplastic type was observed to be more common than the other types among younger people. The clinical signs of hypothyroidism were recognized most frequently in the oxyphilic cell type but not in the hyperplastic type. Thyroid function test, however, demonstrated no definite differences among the four types. Relationships were discussed in this paper between this author's classification and that of Woolner--also relationships to Hashimoto's thyroiditis and Riedel's struma.