The world-wide explosion of overweight people has been called an epidemic. The inflammatory nature of obesity is widely recognized. Could it really be an epidemic involving an infectious agent? In this climate of concern over the increasing prevalence of overweight conditions in our society, we focus on the possible role of oral bacteria as a potential direct contributor to obesity. To investigate this possibility, we measured salivary bacterial populations of overweight women. Saliva was collected from 313 women with a body mass index between 27 and 32, and bacterial populations were measured by DNA probe analysis. Levels in this group were compared with data from a population of 232 healthy individuals from periodontal disease studies. The median percentage difference of 7 of the 40 bacterial species measured was greater than 2% in the saliva of overweight women. Classification tree analysis of salivary microbiological composition revealed that 98.4% of the overweight women could be identified by the presence of a single bacterial species (Selenomonas noxia) at levels greater than 1.05% of the total salivary bacteria. Analysis of these data suggests that the composition of salivary bacteria changes in overweight women. It seems likely that these bacterial species could serve as biological indicators of a developing overweight condition. Of even greater interest, and the subject of future research, is the possibility that oral bacteria may participate in the pathology that leads to obesity.