It is well established that statins display potent anticancer activity in several types of proliferating tumor cells. However, how to promote the sensitivity of statins to mammary cancer is yet to be completely deciphered. The purpose of this study was to investigate whether breast cancer susceptibility gene 1 (BRCA1) overexpression sensitizes mammary cancer cells to statins. MCF-7 cells, which have only one wild-type BRCA1 allele, were transfected with pcDNA3-beta-HA-hsBRCA1 plasmids via liposomes to reconstitute BRCA1 overexpression human breast cancer cell line, and tumoral xenografts with BRCA1 overexpression were subsequently established in BALB/c nude mice. Then, the inhibitory activity of lovastatin on cancer cells and tumoral xenografts, and the underlying mechanism involving in cell-cycle regulatory proteins were analyzed. The proliferative ability of MCF-7 cells treated with lovastatin was reduced compared to normal, and further decreased in the presence of excess BRCA1, detected by methyl thiazolyl tetrazolium and flow cytometry techniques in vitro or by 5-bromodeoxyuridine incorporation in vivo. Additionally, the mRNA and protein expression of cyclin D1, cyclin-dependent kinase 4 (CDK4) and retinoblastoma protein (pRb), was further down-regulated under exposure to lovastatin in condition of BRCA1 overexpression, but the expression of p21WAF1/CIP1, a cyclin-dependent kinase inhibitor (CDKI), was further up-regulated, both in vitro and in vivo detected with quantitative real-time polymerase chain reaction (qRT-PCR) and Western blot analysis. Moreover, we found the further reduced volume of tumoral xenografts treated with lovastatin in the presence of BRCA1 overexpression. Our results suggest that BRCA1 overexpression sensitizes cancer cells to lovastatin via regulation of cyclin D1-CDK4-p21WAF1/CIP1 pathway, which will provide an innovative experimental framework to study control of breast cancer cell proliferation.