The prevalence of antibodies to human T-cell leukemia virus type 1 (HTLV-1), which is linked to the etiology of adult T-cell leukemia (ATL), was examined in 380 patients with various liver diseases in Kumamoto Prefecture, southwestern Japan, which is one of the most endemic area for HTLV-1. Eighteen patients with acute hepatitis (AH), 201 chronic hepatitis (CH), 93 liver cirrhosis (LC) 40 hepatocellular carcinoma (HCC) and 28 with other liver diseases were examined. Among these patients, 110 patients had histories of blood transfusion. HTLV-1 specific antibodies were assayed by the ELISA method and the Western blotting method. The rate of positive reaction was 8.9% in all, 5.6% in AH, 6.0% in CH, 10.8% in LC, 17.5% in HCC and 14.3% in the cases of other liver diseases. The prevalence of anti-HTLV-1 antibodies in about 62,000 healthy blood donors in this area was 4.7%. The overall sero-prevalence in the patient group was significantly higher (p less than 0.001), than in healthy blood donors, particularly in the LC and HCC groups. Although the occurrence increased with age, no difference between sex was observed. Patients who had received blood transfusions were found to have a higher rate (17.2%), than those who had not (5.9%), and healthy blood donors. No difference was found between the two groups regarding family history of liver disease. This study indicates that blood transfusions may be an important route to the HTLV-1 infection.