Heat shock protein 60 expression in the thyrocytes of Hashimoto's thyroiditis and Graves' disease was studied immunohistochemically. Thyrocytes of Hashimoto's thyroiditis showed high expression of heat shock protein 60 not only in tall eosinophilic-cells but also in low and flattened cells, although the former was stained with moderate to strong staining intensity and the latter weakly to moderately. Follicular cells around lymphoid cell follicles were stained more intensely, whereas cells apart from lymphoid cell follicles were stained weakly to moderately. In Graves' disease, only follicular cells around lymphoid cell follicles were stained with varying intensities. Since the pattern in all positive staining was granular, it was thought that heat shock protein 60 over-expressed in thyrocytes located on mitochondria.To investigate the immunological role of overexpression of self heat shock protein 60 in the thyrocytes of Hashimoto's thyroiditis,γσTCR+T-cells in the tissue and the IgG class of anti-self heat shock protein 60 antibodies were studied.γσTCR+T-cells were detected among lymphoid cells scattered in interfollicular connective tissue. No difference in antibody level was seen among subjects with Hashimoto's thyroiditis, Graves' disease and normal subjects. Self heat shock protein 60 overexpression in the thyrocytes of Hashimoto's thyroiditis may play a disease-modifying role, although it does not influence the anti-self heat shock protein 60 antibody level.