OBJECTIVE A minority of acute leukemias have features characteristic of both the myeloid and lymphoid lineages and for this reason are designated mixed-lineage, hybrid or biphenotypic acute leukemias (BAL). There have been difficulties in establishing whether BAL represents a distinct clinico-biological entity due to a lack of objective criteria for distinguishing BAL from acute myeloid leukemias (AML) or acute lymphoblastic leukemias (ALL) with aberrant expression of a marker from another lineage. In this work we analyze diagnostic criteria for BAL. METHODS We describe the features of 26 patients (19 adults and 7 children) with BAL diagnosed at the Royal Marsden Hospital. BAL was defined according to a scoring system devised by our group and the European Group for the Immunological Classification of Leukemia (EGIL). This system is based on the number and degree of specificity of the markers (lymphoid and myeloid) expressed by the blasts. RESULTS According to the FAB criteria, BAL may present as "ALL" or as one of the "AML" subtypes, often M1. It is not infrequent to identify two distinct blast populations: one of small size resembling lymphoblasts and the other larger. The most common immunophenotype is coexpression of B-lymphoid and myeloid markers and less frequently, T-lymphoid and myeloid markers. Cases with a B and T lymphoid phenotype or with trilineage differentiation are rare. BAL has a high incidence of clonal chromosomal abnormalities, the most common being the t(9;22) (q34;q11) (Ph chromosome) and structural abnormalities involving 11q23. Data are emerging that BAL has a negative prognosis in both children and adults and this may be related to the underlying chromosome abnormalities. CONCLUSIONS In summary, BAL is an uncommon type of leukemia which probably arises from a multipotent progenitor cell and carries a poor prognosis. Although there are no uniform criteria about whether to treat these patients as ALL or AML, it is likely that an intensive approach with high-dose therapy followed by bone marrow transplantation will be required to eradicate the disease permanently.