The health effects of human T-lymphotropic virus type I (HTLV-I) were studied in HTLV-I endemic areas in Japan during the period from September 1990 to October 1991. The study subjects were 2,847 men living in the southern part of Kyushu and engaged in light work. Ages ranged from 19 to 79 years (47.8 +/- 7.2; mean +/- standard deviation). The prevalence of each disturbance was analyzed with a logistic model. The disturbances in which the prevalence showed a significant association with HTLV-I were old tuberculosis, ulcerative colitis and ossification of the posterior longitudinal ligament of the cervical spine (OPLL). The results of the present study suggest that HTLV-I may be a risk factor for these diseases and may indicate the necessity for a synthetic study on HTLV-I.