To clarify the cellular composition of subacute thyroiditis, histologic and immunohistochemical studies were performed. Histologically, the lesion presented a patchy distribution of non-caseous granulomas comprising colloid, small lymphocytes, neutrophils, macrophages with or without epithelioid features, and multinucleated giant cells of foreign body type. In addition, numerous plasmacytoid monocytes were closely associated with the granulomas. The giant cells were CD68+, thyroglobulin- and cytokeratin-. Usually, small lymphocytes in the granulomas are CD3+, CD8+, CD45RO+ cytotoxic T-cells. In the non-granulomatous lesion, the follicles were often infiltrated by CD8+ T-lymphocytes, plasmacytoid monocytes and histiocytes, resulting in disrupted basement membrane and rupture of the follicles. Lymphoid follicles with or without active germinal centers were not observed. Moreover, no residual follicular dendritic cell networks were detected by CD23 and CAN.42 immunostains. In the interfollicular area, scattered plasma cells were observed among infiltrating cells. Neither human herpes virus 8 nor EBER-positive cells were detected in the six patients. The findings of our study suggest that cellular immune response may play an important role in the pathogenesis of subacute thyroiditis.