Paclitaxel is used to treat breast cancers, but the mechanisms by which it induces apoptosis are poorly understood. Consequently, we have studied the role of the FoxO transcription factors in determining cellular response to paclitaxel. Western blotting revealed that in a panel of nine breast cancer cell lines expression of FoxO1a and FoxO3a correlated with the expression of the pro-apoptotic FoxO target Bim, which was associated with paclitaxel-induced apoptosis. In MCF-7 cells, which were paclitaxel-sensitive, the already high basal levels of FoxO3a and Bim protein increased dramatically after drug treatment, as did Bim mRNA, which correlated with apoptosis induction. This was not observed in MDA-231 cells, which expressed low levels of FoxOs and Bim. Gene reporter experiments demonstrated that in MCF-7 cells maximal induction of Bim promoter was dependent on a FoxO binding site, suggesting that FoxO3a is responsible for the transcriptional up-regulation of Bim. Gene silencing experiments showed that small interference RNA (siRNA) specific for FoxO3a reduced the levels of FoxO3a and Bim protein as well as inhibited apoptosis in paclitaxel-treated MCF-7 cells. Furthermore, siRNA specific for Bim reduced the levels of Bim protein and inhibited apoptosis in paclitaxel-treated MCF-7 cells. This is the first demonstration that up-regulation of FoxO3a by paclitaxel can result in increased levels of Bim mRNA and protein, which can be a direct cause of apoptosis in breast cancer cells.