Retroocular fibroblast proliferation is believed to be a key component in the pathogenesis of Graves' ophthalmopathy. In the present study, we assessed the ability of superoxide radicals, generated using the xanthine oxidase/hypoxanthine system to induce cellular proliferation in cultured human retroocular fibroblasts obtained from two patients with severe Graves' ophthalmopathy and two control patients undergoing corrective eye surgery. In tissue obtained from patients with Graves' ophthalmopathy, fibroblast proliferation, as assessed by [3H]-thymidine incorporation, was induced by superoxide radicals in a dose-dependent manner. Xanthine oxidase or hypoxanthine alone had no proliferative effect, and control retroocular fibroblasts showed no proliferation in response to superoxide generation. Preincubation with the antithyroid drug methimazole, at concentrations ranging from 0-25 microM, prevented superoxide-induced fibroblast proliferation in a dose-response pattern. Preincubation with the xanthine oxidase inhibitor, allopurinol (1.0 mM) or the antioxidant nicotinamide (10 microM) also inhibited superoxide-induced fibroblast proliferation, whereas propylthiouracil (10 microM) had little effect. These studies suggest a pathway through which oxygen free radicals may contribute to the retroocular fibroblast proliferation observed in patients with Graves' ophthalmopathy.