Social networks and social support are strongly associated with health, yet the pathways between social relations and health are not well understood. In one of the very few studies on this issue conducted in France, we used data from the French GAZEL cohort of employees of the national gas and electricity company, to (1) explore the relationship between the structure and function of the social environment upon self-reported health and (2) test the hypothesis that social relations directly affect health. In a prospective analysis over a 12-month follow-up period, we found that a lack of social support and dissatisfaction with social relations are predictive of poor health status, while weak social networks are not. Thus, functional aspects of social relations are better predictors of poor health than the structure of social interactions. Low social support was an independent risk factor for poor health in men and women: for men the effect was strongest among individuals who held a high occupational status, for women among those in high and low occupational groups. This study suggests that in France social relations exert an independent effect on health, modified by gender and socio-economic factors. Our results indicate that both clinicians in their practice and researchers may do well to focus increased attention on the health impact of social support.