It is well established that obesity is a risk factor for breast cancer and that blood levels of adiponectin, a hormone mainly secreted by white adipocytes, are inversely correlated with the body fat mass. As adiponectin elicits anti-proliferative effects in some cell types, we tested the hypothesis that adiponectin could influence human breast cancer MCF-7 cell growth. Here we show that MCF-7 cells express adiponectin receptors and respond to human recombinant adiponectin by reducing their growth, AMPkinase activation, and p42/p44 MAPkinase inactivation. Further, we demonstrate that the anti-proliferative effect of adiponectin involves activation of cell apoptosis and inhibition of cell cycle. These findings suggest that adiponectin could act in vivo as a paracrine/endocrine growth inhibitor towards mammary epithelial cells. Moreover, adipose adiponectin production being strongly reduced in obesity, this study may help to explain why obesity is a risk factor of developing breast cancers.