The in vivo effect of recombinant human erythropoietin (rHuEpo) and granulocyte 
colony-stimulating factor (G-CSF) combined treatment on CD34(+) cells was 
evaluated by fluorescence in situ hybridization (FISH) in 13 myelodysplastic 
syndrome (MDS) patients with known cytogenetic abnormalities. After treatment, 
responsive patients presented a significantly lower proportion of FISH abnormal 
CD34(+) cells than before treatment (P = 0.003), and in comparison with 
unresponsive cases (P = 0.007). Response to treatment was associated with a 
reduced degree of apoptosis in CD34(+) cells (P = 0.021): however, no difference 
in telomere length was observed in responsive patients after growth factor 
administration. Although the number of patients analysed was relatively small, 
the present data suggest that, in MDS patients, response to rHuEpo and G-CSF may 
be related to the proliferation of karyotypically normal but potentially 
defective CD34(+) progenitor cells.