In the bone marrow (BM) nucleated differential cell count (NDC), myeloblasts are enumerated as a percentage of total nucleated cells, which are inevitably diluted with peripheral blood nucleated cells (PBNC) during BM aspiration. We propose a partial NDC (PNDC) comprising only immature haemopoietic cells capable of division, i.e. myeloblasts, promyelocytes, myelocytes and erythroblasts. We show that the myeloid : erythroid (M : E) ratio of the PNDC remains approximately constant in progressively dilute aliquots of BM aspirates. We determined the PNDC in 22 healthy subjects and investigated the effect of peripheral blood dilution on disease stratification of 66 BM aspirates with myelodysplastic syndromes (MDS). NDC and PNDC myeloblast counts were compared and the equivalent PNDC myeloblast counts for NDC myeloblast threshold counts of 5, 10 and 20% were derived. Reclassification of MDS samples with the PNDC resulted in a change in disease category in 33.3% of 51 MDS samples with NDC myeloblast counts ranging from 3 to 26%. The PNDC is independent of PBNC dilution and can be determined in dilute BM samples. It alters the disease category in a significant proportion of BM aspirates with MDS and has the potential to better stratify MDS to improve clinical outcomes and treatment.