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The nature and extent of relationship between socioeconomic and behavioral variables and body mass index [BMI = weight (kg)/height (m)2] is studied in a sample of 1119 individuals (456 males and 663 females), aged 18 to 75 years, drawn from socioeconomically diverse populations from southern Andhra Pradesh, India. These populations are categorized into four groups, with graded lifestyles toward urbanization. The BMI of the participants ranged from 12.6 to 35.1 in males and from 12.3 to 34.2 in females. There is an increasing trend in mean BMI until about 50 years, followed by a decline, indicating nonlinear nature of age effects. Mean BMI also increases with better socioeconomic status of the constituent groups and with decreased physical activity level. A trend of a decrease in the proportion of individuals with chronic energy deficiency and an increase in the proportion of obese individuals is also seen from the traditional Yerukala tribe (group 1) to the urbanized group 4, from the lower to the higher income categories, and from heavy to light physical activity types. Smokers show a greater proportion of obese cases compared with nonsmokers. The prevalence of obesity (BMI > 25) is 6.6% in males and 10% in females. The results of the analysis of variance suggest that three of the four socioeconomic and behavioral variables (except smoking) show significant effects on age-adjusted BMI, and the R2 suggests that these variables explain 27.4% of variation in males and 17% in females. Although income explains the largest amount of variation (24%) in males, followed by physical activity and group affiliation, in females group affiliation (12.5%), followed closely by income, accounted for most of the variation. Inclusion of age in the model improved explanatory power by 5-7%. The positive association between socioeconomic status and BMI observed in the present study is qualitatively different from the negative association that characterizes contemporary Western populations.