Human T-lymphotropic virus type 1 (HTLV-1) carriers may develop severe systemic diseases, such as adult T cell leukaemia (ATL) or HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). This study aims to investigate systemic diseases of HTLV-1 carriers who had developed HTLV-1-associated uveitis (HAU). We investigated the occurrence of systemic diseases in 200 patients with HAU by performing a retrospective investigation of their medical records and examining the results of a postal survey. The mean age of HAU onset was 49 years, and the total person-years from HAU onset was 1627. There were two cases of ATL. Of these, one was diagnosed with smouldering ATL at the time of HAU onset and the other developed acute-type ATL 4 years after HAU onset. There were 26 cases of HAM/TSP; of these, HAM/TSP occurred first in 13 cases and HAU occurred first in 11 cases. The interval between the onset of HAM/TSP and HAU ranged from 6 months to 6 years, with no significant difference observed based on whether HAM/TSP or HAU occurred first. Hyperthyroidism was noted in 45 cases and preceded onset in all cases. HAU onset occurred after starting thiamazole treatment, and in two cases HAU recurred each time thiamazole treatment was restarted. HTLV-1 carriers with HAU may develop HAM/TSP more frequently than general carriers. HTLV-1 carriers undergoing treatment for hyperthyroidism may be prone to developing HAU.