To elucidate the clinical and epidemiologic features of HTLV-I associated uveitis (HAU), a multicenter case-control study was performed by collaboration of university hospitals throughout Kyushu and Okinawa and two university hospitals in the central metropolitan area. A total of 426 cases of endogenous uveitis were collected and studied between September 1992 and January 1993; about half of the cases were definable for etiology or clinical entity, and the remaining cases were unknown. Assessment of the serum antibodies to HTLV-I revealed that the group of entity-undefined uveitis had a significantly high prevalence of HTLV-I as compared with the age- and sex-matched control subjects, giving supportive evidence for HAU. The titer of serum HTLV-I antibodies was significantly higher in entity-undefined uveitis than in HTLV-carriers. Assuming that a collection of 50 cases of HTLV-I seropositive, etiology-undefined uveitis represents HAU, its clinical features consisted were: (1) middle-aged, otherwise healthy adults developed acute inflammatory uveal disease and presented with visual haze and/or floaters; (2) the disease showed granulomatous or nongranulomatous anterior uveal reactions accompanied by vitreous opacities and retinal vasculitis; (3) the lesions resolved in response to topical or systemic corticosteroids; (4) the visual outcome was usually favorable; (5) nearly half of the cases had recurrent disease; (6) the cases remained systemically unremarkable, except for two cases of HTLV-I associated myelopathy and eight cases of hyper thyroid disease.