To evaluate whether leptin plays a putative role in breast tumorigenesis, we studied the expression of its long and short receptor isoforms in various tumoral breast tissues. We applied semiquantitative RT-PCR method to RNA extracted from 20 breast cancer biopsies and two human breast cancer cell lines (T47-D and MCF-7). Our results showed the expression of both leptin receptor transcripts in all tumoral tissues examined. By in situ hybridization experiments, we localized leptin receptors in proliferating epithelial cells. Study of leptin effects on human breast cancer cells growth was performed by 3H-thymidine incorporation method and colorimetric MTT assay. We demonstrated that leptin (50–100 ng/ml) significantly stimulates proliferation of the human breast cancer cell line T47-D ( P<0.05). Western blot analysis indicated that leptin induces a time-dependent activation of mitogen-activated protein kinases (MAPKinase) 1 and 2 in T47-D cell line. Moreover, the specific MAPK-inhibitor PD 98059 blocked cell proliferation induced by leptin. In conclusion, we demonstrate that leptin receptors are expressed in breast cancer and that leptin induces proliferation in the T47-D cell line via activation of the MAPKinases pathway. These data suggest that leptin and its receptors may be implicated in mammary cell proliferation and in breast cancer pathogenesis.