Human T-cell lymphotropic virus type I (HTLV-I) is a human retrovirus that can transform T-helper lymphocytes and is etiologically associated with adult T-cell lymphoma/leukemia. Mycosis fungoides represents a primary cutaneous lymphoma of helper T-cell origin, while chronic lymphocytic leukemia is generally considered to be a neoplastic B-lymphocyte disorder. Our patient had HTLV-I with coexistent mycosis fungoides and B-cell chronic lymphocytic leukemia. The concurrent lymphoid proliferations may represent HTLV-I-associated abnormalities of immunoregulation.