The July 26 issue of the New England Journal of Medicine (see http://cme.nejm.org/) includes a paper by Christakis and Fowler from Harvard Medical School that reports on a social network analysis of the data of the Framingham Study (see http://www.nhlbi.nih.gov/about/framingham/). The paper shows that people's likelihood to become overweight is much higher when their friends, siblings and/or spouses become overweight. Neighbours' weight changes appeared to have no effects. The study thus suggests that obesity spreads in social networks. Recent reviews of the literature (see for example Ferreira et al. Obesity Reviews 2007;8(2):129-54 [ http://www.blackwellpublishing.com/journal.asp?ref=1467-7881&site=1] and Van der Horst et al. Health Education Research2007;22(2):203-26 [http://her.oxfordjournals.org/]) suggest that social environmental factors are important determinants of physical activity and nutrition behaviours. These studies suggest that more and better effort are needed to use social network approaches in obesity prevention. Promising initiatives such as the La Ville Sante (see http://www.teachers.tv/video/119 for a video presentation on this initiative) use a strong community approach with social networking to promote health behaviours in order to help people to prevent unnecessary weight gain.