Dendritic cell (DC) differentiation was investigated in samples from two acute promyelocytic leukaemia (APL) patients with classic translocation t(15;17)(q22;q21). After 18 d of culture in the presence of granulocyte-macrophage colony-stimulating factor, interleukin 4 and tumour necrosis factor alpha, 10-15% of pathological promyelocytes had differentiated into DC-like cells, as demonstrated by immunological and functional characteristics and by analysis of CD1a+ cells. In one patient, analysed at relapse and after developing a picture of secondary myelodysplastic syndrome (MDS), three different populations of DCs were demonstrated, two of which derived from pathological myeloid precursors (the APL and the MDS clones). This patient's DCs also presented abnormal dextran uptake. Our results demonstrated that pathological myeloid precursors in APL can differentiate into DC-like elements and that different populations of pathological DCs may coexist in the same patient.