Obesity is a risk factor of breast cancers. As leptin, a hormone mainly secreted by white adipocytes, elicits proliferative effects in some cell types. we tested the hypothesis that leptin could influence human breast cancer MCF-7 cell growth. Here we show that MCF-7 cells express leptin receptors and respond to human recombinant leptin by STAT3 and p42/p44 MAPkinase activations and by increased proliferation. These findings suggest that leptin could act in vivo as a paracrine/endocrine growth factor towards mammary epithelial cells thus contributing to explain why obesity is a risk factor of developing breast cancers. (C) 2002 Elsevier Science (USA). All rights reserved.