Recently described immunological functions for heat shock proteins (HSPs) and our previous demonstration of site-selective HSP-72 expression in cultured fibroblasts derived from extrathyroidal manifestations of Graves' disease (GD) prompted us to determine whether expression of the inducible 72-kilodalton HSP can be detected in human thyroid tissues. Immunohistochemistry was performed on formalin-fixed paraffin-embedded thyroid tissue specimens from patients with GD, Hashimoto's thyroiditis (HD), and multinodular goiter (MNG) as well as on normal thyroid tissue. A mouse monoclonal anti-HSP-72 antibody and an ultrasensitive avidin-biotin-peroxidase complex detection system were used for these studies. Striking differences in HSP-72 immunoreactivity were detected both between tissues from GD and HD compared with MNG and normal thyroid and between GD thyroid glands treated preoperatively with antithyroid medication and untreated GD glands. Strong HSP-72 reactivity in GD and HD tissues was detected in thyroid follicles as well lymphocytic infiltrates. No HSP-72 reactivity was detected in MNG or normal thyroid tissue. HSP-72 immunoreactivity was markedly reduced in GD glands that received preoperative antithyroid drug treatment. In conclusion, high levels of HSP-72 expression in autoimmune thyroid disease may reflect a state of chronic cellular stress, but could also represent an immunomodulatory factor of relevance in the autoimmune process in GD.