We investigated serum levels of interleukin-5 (IL-5) in order to examine the role of T-helper 2 (Th2)-type immune response in the pathogenesis of autoimmune thyroid diseases. Serum levels of IL-5 were determined by a highly sensitive sandwich enzyme-linked immunosorbent assay in 42 patients with Graves' disease, 32 patients with Hashimoto's thyroiditis, 12 patients with silent thyroiditis, and 21 normal controls. Compared with serum levels in normal subjects (5.8 +/- 4.2 pg/mL), IL-5 was increased in patients with Graves' disease (16.4 +/- 16.7 pg/mL, p < .01), and in patients with Hashimoto's thyroiditis (10.0 +/- 7.6 pg/mL, p < .05), but not in patients with silent thyroiditis. There was no correlation between serum free thyroxine (FT4) and IL-5 levels. These data suggest an important role of the Th2-type immune response in the pathogenesis of Graves' disease and Hashimoto's thyroiditis.