Adult T-cell leukemia/lymphoma is a rare malignancy associated with the human retrovirus human T-cell lymphotropic virus type 1. It is characterized by the proliferation of highly pleomorphic lymphocytes. Involvement of peripheral blood, bone marrow, lymph nodes, spleen, and extranodal sites such as skin, liver, gastrointestinal tract, and central nervous system can occur. There are four distinct clinical variants, and the prognosis and clinical course range from highly aggressive to a more protracted course depending on the subtype. We describe a man with de novo adult T-cell leukemia/lymphoma and discuss the unique clinical, morphologic, immunophenotypic, and molecular features of this entity.