Adult T-cell leukemia/lymphoma (ATLL) is a human malignancy associated with the retrovirus, human T-cell leukemia virus type I (HTLV-I). In a previous series of studies, our group and others characterized the histopathological changes in HTLV-I-associated lymph node lesions. In addition to the pleomorphic and anaplastic large cell types of typical ATL lymphoma, we identified lymph nodes with an unusual Hodgkin's disease-like histology (Hodgkin-like ATLL) in HTLV-I-positive patients, with Hodgkin-like ATLL showing prodromal clinical features. We also reported HTLV-I-associated lymphadenitis, characterized by non-neoplastic HTLV-I-associated lymph node lesions. It has become clear that the biological and clinical behavior of malignant lymphoma is not only determined by the properties of the lymphoma cells themselves, but also largely by the interaction of these cells with their nonmalignant microenvironment. In this review, we discuss the pathological variations of microenvironments, which are important for clarification of the histological features associated with HTLV-I.