OBJECTIVE To identify the possible association between dental caries and body mass index (BMI) and to explore the effect of BMI on tooth eruption in a cohort of elementary schoolchildren. METHODS A 4-year longitudinal study was completed. A total of 110 children from a public elementary school, located in a middle-income area of Mexico City, entered the study; of these, 88 completed the 4-year follow-up period. Dental caries assessments were carried out using the WHO criteria for decayed, missing and filled primary and permanent teeth indices (dmft and DMFT, respectively) and surface indices (dmfs and DMFS, respectively). BMI was used to classify the children's obesity status, according to the Centers for Disease Control 2000 reference charts. RESULTS At 7 years of age, 29.6% of the children were in the overweight or at risk of being overweight categories and, by 11 years of age, this proportion had risen to 45.5%. The mean dmft for children aged 7 years was 2.70 and, for children aged 11 years, the DMFT was 0.54. Children in the higher BMI categories had more erupted teeth than the other children (p < 0.001). A lower dmfs index was detected in the overweight children, compared with children with a lower BMI (p < 0.001). CONCLUSIONS The overweight children had more erupted teeth and a lower caries index. The complex relationships between body composition and oral health should be considered in pediatric patients.