Most cases of acute leukemia can be assigned to the myeloid, B or T lineage. In a few cases, definitive assignment cannot be achieved because blasts express antigens of more than one lineage. A subset of these, referred to as acute bilineal leukemias (aBLLs), is characterized by the presence of more than one population of blasts, each comprising a single lineage. We identified 19 cases of aBLL, including 10 mixed T and myeloid (T-My) and nine mixed B and myeloid (B-My); no mixed B and T cases were identified. Cytogenetic data were available for 16 patients. Three of seven patients with B-My had a t(9;22)(q34q11.2), two had 11q23 translocations and one had del(9). Two of nine patients with T-My had 2p13 translocations; five had other unrelated abnormalities. Of 16 patients with outcome data, only six achieved complete remission and only two remain free of disease 2.5 and 4.5 years after chemotherapy or stem cell transplantation. aBLL is a rare disease that combines B or T and myeloid blasts. Cytogenetic abnormalities of t(9;22) and 11q23 are common in, and may be restricted to, B-My cases, while T-My cases have frequent but generally non-recurring abnormalities. Both types of aBLL are associated with poor outcome.