We studied the cellular localization of heat shock protein 70 (HSP70) synthesis in human orbital tissue from patients with Graves' ophthalmopathy(GO) before and after immunosuppressive therapy. Synthesis was detected using in situ hybridization techniques. In a specimen from a GO patient in active phase, strong expression of HSP70 mRNA was detected in infiltrating lymphocytes as well as in fibroblasts. HSP70 mRNA was also detected in orbital tissue fibroblasts from GO patients following treatment, but not in control specimens. HSP70 expression in the orbital tissue of GO patients may be important in mediation of the immune process.