The study reported here explored the associations of body mass index (BMI), socio-economic status (SES), and beverage consumption in a very low-income population. A house-to-house survey was conducted in 2003 of 12,873 Mexican adults. The sample was designed to be representative of the poorest communities in seven of Mexico's 31 states. Greater educational attainment was significantly associated with higher BMI and a greater prevalence of overweight (25 < or = BMI<30) and obesity (30 < or = BMI) in men and women. The combined prevalence of overweight and obesity was over 70% in women greater than the median age of 35.4 years with at least some primary education compared with a prevalence of 45% in women below the median age with no education. In both sexes, BMI was positively correlated with education, occupation, quality of housing conditions, household assets, and subjective social status. BMI and household income were significantly correlated in women but not in men. In the models including all SES variables, education, occupation, housing conditions and household assets all contributed independently and significantly to BMI, and household income and subjective social status did not. Increased consumption of alcoholic and carbonated sugar beverages was associated with higher SES and higher BMI. Thus, in spite of the narrow range of socio-economic variability in this population, the increased consumption of high calorie beverages may explain the positive relationship between SES and BMI. The positive associations between SES and BMI in this low-income, rural population are likely to be related to the changing patterns of food availability, food composition, consumption patterns and cultural factors. Contextually sensitive population-level interventions are critically needed to address obesity and overweight in poor populations, particularly in older women.