A case of acute leukaemia with t(4;l 1) chromosomal abnormality in a 28-year-old woman is reported. At diagnosis, two blast cell populations were seen: 60% of the cells were small cells with lymphoid morphology, 40% were large cells with monocytic morphology. Cytochemical examination was consistent with acute myeloid leukaemia (peroxidase-positive in 10% of the cells), but surface markers were those of common acute lymphoblastic leukaemia (CALLA, B4, TdT-positive, but My7-, My9- and OKM1-negative). Five days after diagnosis, although the only treatment had been platelet transfusions, there was a change in morphological and immunological phenotype: 40% of the cells were lymphoid and 60% monocytic. Lymphoid markers were expressed in only 20–40% of cells, and myeloid markers appeared on up to 60% of cells. We conclude that t(4;l 1) leukaemia could originate in an undifferentiated progenitor cell, which can undergo further differentiation into lymphoblasts or monoblasts, and that we were able to observe this in vivo differentiation in our patient.