The diverse clinical manifestations of 10 cases of so-called adult T-cell leukemia-lymphoma (ATL)-related T-cell malignancies are described. These cases were anti-ATLA [antibody to ATL virus (ATLV)-associated antigen (ATLA)] positive, and tumor cells had the inducer/helper T-cell phenotype and expressed ATLA when cultured, indicating that these diseases are the same as typical ATL, even though their clinical diagnoses were different from ATL. Accordingly, they are called atypical ATL. Clinically, they could be divided into two subtypes, smoldering type and lymphoma type. In the smoldering type, the disease usually started with skin lesions and rarely with lung lesions. After a prodromal stage of several years, the disease progressed insidiously to the leukemic stage without additional manifestations. The flower cells characteristic of typical ATL were observed in only a small percentage of peripheral lymphoid cells. In two of the five patients the disease progressed to typical ATL after several years from onset. All five patients are alive with a long survival time, more than 6 yr in four, and had high titers of anti-ATLA, suggesting that anti-ATLA might have some role in regulating the proliferation of ATL cells in vivo. In the lymphoma type, morphological diagnosis was not always specific for discriminating ATL-related from ATL-unrelated T-cell lymphomas. Detection of anti-ATLA in the patient's serum and of ATLA in cultured tumor cells, examination of the sera of members of the patient's family for anti-ATLA, and observation of typical flower cells in the peripheral blood though the patients had neither lymphocytosis nor leukemic changes, seem to be useful for the discrimination, especially in an ATL-nonendemic area. Members of the family of a patient with anti-ATLA positive T-lymphoma in an ATL-nonendemic area were also anti-ATLA positive, indicating that healthy ATLV carriers in an ATL-nonendemic area exist as a family colony. This is responsible for sporadic outbreaks of ATL in an ATL-nonendemic area. In summary, the disease entity of ATL is considered, at present, to be a malignancy of inducer/helper T-cells transformed by ATLV or HTLV (human T-cell leukemia virus). In this sense, diverse clinical manifestations of ATL should be recognized as events of viral oncogenesis and host immune response.