OBJECTIVE The aim of this study was to determine the association between obesity and caries by utilizing the data of a cohort of preschool children aged 4-5 years. METHODS Data were obtained from a cohort of 1,160 children. Dental caries detection was performed according to the World Health Organization criteria. The caries index was measured as the number of decayed (d), extracted (e), and filled (f) teeth (t) (deft), or surfaces (defs). The body mass index (BMI) in units of kg/m2 was determined, and children were categorized according to age- and gender-specific criteria as normal weight (5th-85th percentile), at-risk overweight (> or = 85th-<95th percentile), and overweight (> or = 95th percentile). Odds ratios were determined for at-risk overweight and overweight children using logistic regression. RESULTS The prevalence of dental caries was 17.9 percent. A slightly higher percentage of dental caries was found in boys (19.6 percent) than in girls (16.4 percent). From the total sample, the mean BMI was 17.10 +/- 3.83. Approximately 53.7 percent of children were classified as normal weight, 14.2 percent as at-risk overweight, and 32.1 percent as overweight. At-risk overweight children were higher among girls (17.1 percent) than among boys (11.3 percent). When adjusted for covariates, the logistic regression model showed that there was a significant association between at-risk overweight children (P < 0.001), overweight children (P < 0.001), and caries in the primary dentition. Mean (SD) deft value of the sample was 1.08 (2.34), while the corresponding defs value was 1.43 (3.29). CONCLUSIONS Obesity appears to be associated with dental caries in the primary dentition of preschool Mexican children.