Leptin is produced in the mammary gland by the fat tissue or by the mammary epithelium. The aim of this study was to investigate the role of leptin on mammary epithelial cell differentiation and cell viability. This study was conducted using the mouse mammary epithelial cell line HC11. We show that leptin, synergizes with prolactin to increase beta-casein gene expression during mammary epithelial cell differentiation. This was correlated with increased phosphorylation of the signal transducer and activator of transcription 3 (STAT-3). Inactivating the function of STAT-3 by expression of a short hairpin RNA demonstrated that the effect of leptin on beta-casein expression is mediated by STAT-3. Secondly, cells in which STAT-3 had been inactivated showed increased cell viability compared to controls and were resistant to the negative effect mediated by leptin. Further, leptin triggers apoptosis in mammary epithelial cells cultivated in non-differentiating conditions. Taken together, these results suggest that leptin, by activating STAT-3, may act as a paracrine factor modulating mammary epithelial cell function.