Recently, several cases of adult T-cell leukemia (ATL) with CD30 antigen have been reported, but its clinical significance remains unknown. Accordingly, we studied CD30 antigen expression in ATL cases and documented the clinicopathological characteristics of these cases. Immunohistochemical and clinical characteristics were studied in 46 patients with malignant lymphoma or benign lesions of lymphoid tissue, who had antibodies against human T-cell leukemia virus type I (HTLV-I). Monoclonal integration of HTLV-I provirus was demonstrated in the tumor cells in 36 (ATL) of the 46 cases. CD30 antigen expression was evident in seven of these 36 cases (19.4%), however it was not seen in any of the ten cases lacking the integration of HTLV-I provirus. A comparison of ATL cases with and without CD30 antigen expression revealed significantly larger numbers of abnormal lymphocytes in the peripheral blood and lower serum calcium levels in ATL expressing CD30 antigen.