It has been reported that the adenine nucleoside analogue fludarabine is able to increase the phosphorylation and the cytotoxicity of cytosine arabinoside (Ara-C) in different leukemic models, both in vitro and in vivo. In poor prognosis acute myeloid leukemia (AML), the combination of fludarabine with Ara-C and granulocyte colony-stimulating factor (G-CSF) has proven to be a highly effective regimen. In this study we aimed to further investigate the effects of this drug combination. In vitro, on fresh AML cells from ten patients, our results confirm an additive cytotoxic effect displayed by fludarabine + Ara-C, as demonstrated by isobologram analysis of the data. The addition of G-CSF significantly increased the efficacy of the drug combination. These effects appeared to be related to an increased incorporation of [3H]Ara-C into cellular DNA in the presence of fludarabine + G-CSF. Furthermore, the quantitative evaluation of programmed cell death (apoptosis) showed that fludarabine + Ara-C + G-CSF induce apoptosis to a higher degree than either compound alone. This finding suggests that cooperative induction of apoptosis could be the potential mechanism of action of this drug combination.