The PI3K/Akt signal pathway plays a key role in tumourigenesis of many cancers and in the subsequent development of drug-resistance. Using the K562 chronic myelogenous leukaemia (CML) cell line and the doxorubicin-resistant derivatives lines KD30 and KD225 as models, we observed that enhanced PI3K/Akt activity and acquisition of chemoresistance correlated unexpectedly with increased expression and nuclear accumulation of FOXO3a. Moreover, we found that induction of FOXO3a activity in naܯve K562 cells was sufficient to enhance PI3K/Akt activity and to confer resistance to the cytotoxic effects of doxorubicin. Conversely, knock-down of endogenous FOXO3a expression reduced PI3K/Akt activity and sensitized these cells to doxorubicin. Further chromatin immunoprecipitation (ChIP) and promoter mutation analyses demonstrated that FOXO3a regulates the expression of the PI3K catalytic subunit p110alpha through activation of a promoter region proximal to a novel untranslated exon, upstream from the reported transcription start site of the p110alpha gene PIK3CA. As was the case for FOXO3a, expression or knock-down of p110alpha was sufficient to amplify or reduce PI3K/Akt activity, respectively. Thus, our results suggest that chronic activation of FOXO3a by doxorubicin in CML cells can to enhance survival through a feedback mechanism that involves enhanced p110alpha expression and hyperactivation of the PI3K/Akt pathway.