Spontaneous and mitogen-stimulated production of interleukin-4 (IL-4), IL-6, IL-10, IL-12, interferon-gamma (IFNgamma), and tumor necrosis factor-alpha were evaluated by enzyme-linked immunospot assay of peripheral blood mononuclear cells from patients with Graves' disease immediately before and at 4, 17, and 59 days after treatment with radioactive iodine. Patients had significantly reduced IL-4 and IFNgamma production before treatment compared with healthy controls. Both cytokines were increased to normal levels by day 17 after treatment, and IFNgamma remained at normal levels on day 59, whereas IL-4 returned to subnormal levels at this time. IL-12 production was initially normal and was not significantly altered by therapy. IL-6, IL-10, and tumor necrosis factor-alpha were also normal before radiotherapy, but increased significantly on day 17, returning to pretreatment levels by day 59. Thus, radioiodine treatment induced a transient increase in both proinflammatory and antiinflammatory cytokines and a more prolonged increase in IFNgamma production, the latter representing a definite shift toward a type 1 cytokine profile.