Cytokines have a central role in the generation of an autoimmune response and can directly affect the target organ. In Graves' disease, both the infiltrating mononuclear cells and the thyroid follicular cells produce certain cytokines, but the relative contribution of each is unclear, and there are conflicting data on the exact profile of cytokines expressed within the thyroid. To clarify these issues, we used the method of reverse transcription-polymerase chain reaction amplification to analyze cytokine gene expression by intrathyroidal lymphocytes (ITL) and purified thyroid follicular cells (TFC) from six patients with Graves' disease. All ITL samples were positive for interleukin-1 alpha (IL-1 alpha), IL-1 beta, IL-6, IL-8, IL-10, and tumor necrosis factor-alpha (TNF alpha) messenger ribonucleic acids (mRNAs). Four samples were positive for IL-2 mRNA, and of these, three were also positive for interferon-gamma (IFN gamma). All TFC samples contained IL-6 and IL-8 mRNAs, even after depletion of CD3-positive T-cells. One TFC sample was additionally positive for IL-10 and TNF alpha mRNAs, and in the case of IL-10, this signal was not eliminated by CD3-positive T-cell depletion. IL-4 was not detected in any sample of ITL, TFC, or whole tissue. Semiquantitative analysis showed that the ITL fraction represented the major source of IL-6, IL-8, and TNF alpha mRNAs. By contrast, only three of five multinodular goiter samples were positive for IL-1 alpha mRNA; of these, two were also positive for IL-6, and 1 was positive for IL-8 mRNA. One multinodular goiter sample was positive for IL-8 mRNA alone, but IL-2, IL-4, IL-10, and TNF alpha mRNAS were not detected. These results suggest that although the TFC themselves may express certain cytokines, the ITL population represents the most important source of cytokine production in Graves' thyroid glands. The presence of IL-2, IFN-gamma, and TNF alpha and the absence of IL-4 mRNA in samples of ITL indicate a pattern of cytokine production that most closely resembles that of the TH1 helper T-cell subset. Given the etiological role of thyroid-stimulating antibodies in Graves' disease, the production of which is likely to depend upon TH2 helper T-cell function, it is perhaps surprising that the TH1 subset appears to predominate. It is possible that IL-10 is important in stimulating intrathyroidal autoantibody production, and this cytokine may also play a role in inhibiting cell-mediated thyroid injury in Graves' disease.