Abstract Objective: This study investigated the association between duration of breastfeeding and occurrence of distoclusion in preschoolers with complete deciduous dentition. Subjects and Methods: A cross-sectional study nested within a cohort of 153 Brazilian children was conducted. The outcome of interest was presence of distoclusion as diagnosed at dental assessment between 3 and 5 years of age. Data on the main exploratory variable (duration of breastfeeding) and other variables that might influence the study outcome (use of dummy/pacifier, bottle feeding, and thumb sucking) were collected by a blinded examiner at 7, 30, 60, 120, and 180 days of life and at dental assessment. Poisson regression was used to test for the association between exposure and the outcome of interest. Results: The prevalence of distoclusion was 47.7%. Multivariate analysis showed that breastfeeding afforded some protection against distoclusion, after adjusting for confounding factors such as pacifier use and bottle feeding. In children breastfed for 12 months or more, the prevalence of distoclusion at 3–5 years of age was 56% lower than in those breastfed for shorter periods (prevalence ratio=0.44; 95% confidence interval 0.23–0.82). Conclusion: Breastfeeding for 12 months or longer significantly reduces the prevalence of distoclusion in the deciduous dentition.