We describe a unique case of hybrid leukaemia with bilineal and biphenotypic features. The coexistence of lymphoblasts and monoblasts was determined morphologically and cytochemically. Immunofluorescence and immunohistochemical analysis revealed that each blast population had both T lymphoid (CD2, cytoplasmic CD3) and myeloid (CD11, CD13, CD15) markers. Southern blot analysis of DNA extracted from the lymph node biopsy demonstrated the presence of monoclonal rearrangement of the TcR-C beta gene. Cytogenetic analysis of the bone marrow cells showed a karyotype of 48, XY, 7q+ in all of the metaphases examined. These observations are suggestive of a monoclonal origin for these two distinct blast populations.