The records of 76 consecutive patients with etiology-undefined uveitis examined during the 3-year period between 1990 and 1992 were reviewed and 6 patients were found who had concomitant hyperthyroidism. These 6 patients had presented with uveitis symptoms and signs when hyperthyroidism was relieved with thiamazole therapy. The uveal disease was characterized by acute, granulomatous or nongranulomatous anterior uveal involvement and granular or membranous vitreous opacities with or without retinal vascular change. These manifestations resolved in a few weeks in response to topical and/or systemic corticosteroids. Three patients had recurrence of uveitis after remission for months to years. All of the patients had serum antibodies to HTLV-I. The uveal disease resembled HTLV-I associated uveitis that may develop in patients with HTLV-I associated myelopathy or HTLV-I carriers; 2 cases had chronic myelopathy or arthropathy that was considered associated with the retrovirus. The present observations suggest that the association between uveitis and hyperthyroidism is not accidental but shares a common underlying etiologic factor, HTLV-I, although the pathomechanism remains to be defined.