The FAB classification of acute leukaemias is based on the light microscopic detection of myeloperoxidase (MPO) activity in blast cells. Cases with MPO activity in > or = 3% of blasts are classified as acute myeloid leukaemia. We describe two cases of acute leukaemia in which the blast cells had lymphoid morphology, ultrastructure, immunophenotype and molecular rearrangements, but expressed significant levels of the MPO gene (MPO mRNA) by reverse transcription polymerase chain reaction (RT-PCR), in the absence of translation to protein. Both cases presented a short remission duration. We discuss the incidence, features and outcome of MPO mRNA positive acute lymphoblastic leukaemia (ALL).