BRCA1 germline mutations predispose women to early onset, familial breast and ovarian cancer. BRCA1 has been recently implicated in the cellular response to agents that disrupt the mitotic spindle. In this report, we studied BRCA1 contribution to paclitaxel response in MCF-7 breast cancer cells. We show that MCF-7 cells transfected with BRCA1 siRNA display a significant increase in resistance to paclitaxel compared with the control cells. We next demonstrate that downregulation of BRCA1 reduces the mitotic index and triggers premature cyclin B1 degradation and decrease in Cdk1 activity following paclitaxel treatment, suggesting that BRCA1 downregulation results in precocious inactivation of the spindle checkpoint. These findings were confirmed by showing that BRCA1 downregulation induces premature sister-chromatids separation in MCF-7 cells following spindle damage. Furthermore, we show that BRCA1 up-regulates the expression of the protein kinase BubR1, essential component of the functional spindle checkpoint, whose downregulation is known to result in paclitaxel resistance in MCF-7 cells. Altogether, our findings support the notion that downregulation of BRCA1 expression mediates paclitaxel resistance through premature inactivation of spindle checkpoint in MCF-7 breast cancer cells. They link BRCA1 to the mitotic checkpoint that plays an essential role in the maintenance of chromosomal stability.