OBJECTIVE: To measure the relationship between childhood obesity and dental caries in preschool children, by means of two anthropometric growth curves. METHODS: A cross-sectional study of caries was conducted in 119 children in the age range from 3 to 5 years, resident in shantytowns in São Paulo, SP, Brazil. Clinical oral exams, nutritional evaluation and record of frequency of food, behavioral and social data collection were performed. For dental caries the dft index (carious and filled teeth)) was used. In the nutritional evaluation, the National Center of Health Statistics – NCHS classification was used, and World Health Organization – WHO reference with regard to dietary intake, the frequency of food record was used. To evaluate the data, logistic regression analysis was used. RESULTS: Obese children presented a high prevalence of dental caries of 81%, higher than the other groups, when evaluated by the NCHS criteria, and for this parameter they presented Prevalence Ratio - PR of 1.23 IC95% (1.00 – 1.50) for dental caries, frequency of food PR 0.94 IC95% (0.76 – 1.17), maternal schooling PR 1.07 IC95% (0.81–1.40) and visits to the dentist PR 1.03 IC95% (0.78-1.37). CONCLUSIONS: For the NCHS criterion there was association of dental caries with obesity, irrespective of the other factors evaluated. No association was found when evaluated by the WHO. Caution is needed on the decision about the method for nutritional diagnosis.