In this report we describe an Italian patient with chronic T cell leukemia whose proliferating cells were mature T lymphocytes with a helper phenotype (T helper phenotype chronic lymphocytic leukemia, or Thp-CLL). Unlike other reported cases of Thp-CLL, fresh leukemic cells from this patient were positive with the anti-Tac monoclonal antibody, which recognizes the receptor for interleukin-2 (IL-2). Thus, the phenotype of these cells was similar to that expressed by Japanese patients with adult T cell leukemia (ATL) (OKT3+, OKT4+, OKT8-, Tac+). However, the Italian patient had Thp-CLL, not ATL, since his cells, unlike ATL cells, lacked human T cell leukemia virus (HTLV-I)-related DNA sequences. The Tac receptor, which appears to be modulated in vitro by the anti-Tac antibody, was biologically inactive, since the patient's cells did not respond in vitro to IL-2. In addition, they also failed to demonstrate in vitro functional activities. The clinical course was aggressive, as usual, for both Thp-CLL and ATL. Taking advantage of the description of this case, some similarities and differences between Thp-CLL and ATL are discussed, focusing on the importance of screening of HTLV-I in the differential diagnosis.