Biphenotypic acute leukaemia with T-lymphoid and myeloid markers is rare and poorly documented. In the Leucemie Aigue Lymphoblastique de l'Adulte (LALA) prospective trial (LALA 94) of treatment for adult acute lymphoblastic leukaemia (ALL), seven patients (0.86%) had T-biphenotypic forms. The clinical and biological characteristics and outcome of these seven patients are reported here. The patients' median age was 35 years. At diagnosis, all had a tumoural syndrome and five had a mediastinal mass. In all the cases, leukaemic cells expressed myeloid and lymphoid markers. Two patients (28%) entered complete remission (CR) after induction chemotherapy. Four of the five remaining and assessable patients entered CR after designed salvage chemotherapy with mitoxantrone and high-dose cytosine arabinoside. Three patients are currently in CR. Three patients died, from treatment toxicity in two cases and progressive disease in one case. One patient relapsed 6 months after allogeneic bone marrow transplantation and is still alive. Thus, biphenotypic T-acute leukaemia is clinically frequently associated with mediastinal involvement and the response to conventional chemotherapy used in ALL is poor. However, sustained CR can be achieved by salvage chemotherapy combining an intercalating agent with high-dose cytosine arabinoside, as used in acute myeloid leukaemia.