Using a multiparameter flow cytometry assay enumerating cells positive for CD13, CD14 and CD66 antigens, we determined the asynchronous CD14/CD66 co-expression in unselected bone marrow and peripheral blood samples with suspected malignant blood disorders. CD14/CD66 co-expression ≥5% were found in 131/691 bone marrow samples. Only 55 of these exhibited an identifiable population in 2-parameter flow cytometry histograms. Of the 55 samples 43 (78%) came from patients with myeloid disorders; e.g. 11 with myelodysplastic syndromes, 15 with chronic myeloproliferative disorders and 17 with acute myeloid leukaemia. Only one of these 17 cases was a de novo case, while 8 were secondary to another malignant haematological disease and 8 were from the period after cytoreductive therapy. Notably, CD14/CD66 co-expression patterns were related to disease categories; e.g. in chronic myelomonocytic leukaemia and acute myeloid leukaemia following a dysplastic phase the co-expression displayed two subsets in peripheral blood, low-avidity CD14 and low-avidity CD66, respectively. The latter disease category also exhibited these 2 subsets in bone marrow. In all other cases, the CD14/CD66 co-expression in bone marrow was heterogeneous. In conclusion, abnormal CD14/CD66 co-expression might be a valuable parameter in defining asynchronous myelopoiesis in malignant myeloid disorders, especially myeloproliferative disorders and secondary acute myeloid leukemias.