TAO is characterized by an autoimmune process affecting the orbital contents. T cells have been suggested to have a major role in pathogenesis, but so far only limited data are available to clarify the extraocular muscle (EOM)-infiltrating T cell phenotype, antigenic reactivity and cytokine profile in TAO patients. In the present study, biopsies of affected EOM were taken and the infiltrating T cells isolated and expanded in vitro with mitogen. Their phenotype was determined by flow cytometric (FACS) analysis and compared with peripheral blood-derived T cell lines, treated in the same way from the same patient. Cytokines present in the supernatant after mitogen stimulation of the T cell lines were assayed by ELISA. In addition, cytokine mRNA present at the time of biopsy was determined by rapid RNA extraction from EOM and reverse transcription-amplification with specific cytokine oligonucleotide probes (IL-1alpha, IL-1beta, IL-2, IL-4, IL-6, IL-8, IL-10, IL-12, IL-13, IL- 15, interferon-gamma (IFN-gamma), tumour necrosis factor-alpha (TNF-alpha)). In the T cell lines from two patients, proliferation assays were carried out with antigens derived from thyroid gland, EOM and a thyrotropin (TSH) receptor preparation. Most T cell lines were CD4+, CD45RO+, and TCR alpha/beta+, both from the EOM and the peripheral blood. A wide variety of cytokines was detected by analysis of supernatants or mRNA, but the profiles were not identical comparing the two approaches. However, IL-4 was detected by both. Dose-dependent proliferation was observed in response to thyroid extract in a biopsy-derived T cell line. In conclusion, EOM-infiltrating T cells from patients with TAO, expanded in vitro, were chiefly CD4+ and produced a mixture of cytokines, including IL-4. The proliferation data suggest that there are thyroid-reactive T cells in EOM.