BACKGROUND AND OBJECTIVES: Breastfeeding is an evidence-based recommendation for all countries, but breastfeeding rates have been declining in many middle-income settings. One reason behind this decline is the perception that breastfeeding may not be necessary in modern urban settings, where clean water is available and alternative foods are abundant. We investigate the importance of breastfeeding for early childhood development in the modern urban context of São Paulo, Brazil. METHODS: In our study, we used data from the ongoing prospective Western Region Birth cohort in São Paulo, Brazil. Children were recruited at birth and managed for 3 years. Durations of exclusive and mixed breastfeeding were our primary independent variables. Our secondary independent variable was an indicator for compliance with World Health Organization (WHO) breastfeeding recommendations. Our primary outcomes of interest were indicators of children’s physical, cognitive, language, and social-emotional development at 3 years of age. Adjusted estimates and 95% confidence intervals were calculated by using linear and logistic regression. RESULTS: Complying with WHO recommendations to exclusively breastfeed for 6 months followed by complementary feeding until 2 years of age was associated with a 0.4-SD increase in overall child development (β: .38; confidence limit = 0.23 to 0.53), a 0.6-SD increase in height-for-age z score (β: .55; confidence limit = 0.31 to 0.79), and a 67% decrease in the odds of stunting (odds ratio = 0.33; 95% confidence interval = 0.20 to 0.54). CONCLUSIONS: Our results suggest that even in settings with easy access to complementary foods, complying with WHO breastfeeding recommendations is important for healthy physical growth and cognitive development.