BACKGROUND AND OBJECTIVE: Although biphenotypic leukemia is now a defined entity, outcome of this rare form of acute leukemia has not been well documented. We present the first comprehensive study analyzing induction and consolidation therapy of biphenotypic leukemia and correlate outcome to prognostic factors. DESIGN AND METHODS: In this retrospective study, the incidence of biphenotypic leukemia was found to be 3.6% from 693 adult and pediatric acute leukemias referred to our center for treatment over the last 8 years. Of these, 15 were B-lymphoid/myeloid, 8 were T-lymphoid/myeloid, one was T/B lymphoid and one had trilineage differentiation. RESULTS: Induction of remission in de novo cases was achieved in 70% of patients and relapse of disease occurred in 15%. The use of combined lymphoid and myeloid drugs for induction resulted in a high incidence of early deaths (25%). The overall probability of survival at 2 years was 39.4%. Patients with secondary disease had a uniformly poor outcome with low remission rates and high relapse rates. INTERPRETATION AND CONCLUSIONS: Prognosis was most strongly related to the presence of the Philadelphia chromosome (p=0.03) and age under 15 years (p=0.01). We conclude that patients with biphenotypic leukemia should have risk stratification with treatment tailored to their prognostic factors.