In order to clarify the prevalence of human T-cell leukemia virus type I (HTLV-I) infection in the Kagoshima district, Japan, a highly endemic area for HTLV-I, antibodies for HTLV-I (anti-HTLV-I) were examined in the sera of 6167 from healthy residents and patients with various hematologic and nonhematologic diseases. In healthy residents, including blood donors, the prevalence of anti-HTLV-I was 11.9% (562/4741 persons). The prevalence increased with age, and was significantly higher in in females than in males (P less than 0.01). The prevalence of anti-HTLV-I in blood donors was 8.5%. In In hematologic diseases, the prevalence of anti-HTLV-I was 98.3% in ATL, 28.9% in lymphoproliferative disorders except ATL, and 10.6% in myeloproliferative disorders. In nonhematologic diseases, the prevalence of anti-HTLV-I was shown to be 29.5% in pulmonary tuberculosis, 25.8% in leprosy, 33.8% in chronic renal failure (CRF), 21.9% in autoimmune diseases, and 47.8% in strongyloidiasis. The various diseases except myeloproliferative disorders had significantly higher prevalence of anti-HTLV-I than healthy residents (P less than 0.01 or 0.05). For autoimmune diseases, the prevalence of anti-HTLV-I in patients with blood transfusion (55.6%) was higher than in those without blood transfusion (8.7%), and healthy residents. In hemodialysis patients with CRF who had received blood transfusions the prevalence of anti-HTLV-I increased with the number of blood transfusions. Therefore, HTLV-I transmission via blood transfusion would partially explain these high prevalence of anti-HTLV-I. However, the prevalence of anti-HTLV-I in hemodialysis patients with CRF was statistically higher than that in healthy residents, regardless of blood transfusion (P less than 0.01). Furthermore, hemodialysis patients showed significantly higher prevalence of anti-HTLV-I than healthy residents, even at a younger age. Patients with pulmonary tuberculosis and leprosy showed the same results as hemodialysis patients. These results suggest that possibility that HTLV-I infection has some relation not only to ATL but also to other diseases. Therefore, it seems very important to halt the spread of HTLV-I transmission as soon as possible.