OBJECTIVE: To verify the prevalence of breastfeeding in the city of São Carlos. METHOD: For the collection and treatment of data regarding prevalence of breastfeeding we used the LACMAT 3.3 program. During Vacination National Day (1998, August 15) 3,326 persons responsible for children 2 years old or younger were interviewed. RESULTS: It was verified that 52.4% of the children under a month old were on exclusive breastfeeding. Out of 532 children under 4 months old, 73.3% were being breast fed: 37.8% exclusively breast fed and 17.3% under predominant breastfeeding. It was observed that 31.7% of the children under 4 months received some other form of nourishment, such as fruit and mush, and on the fifth month, this percentage went to 62.3%. CONCLUSIONS: The results of this study demonstrated that the situation of breastfeeding in São Carlos is far from what the WHO recommends, this confirming the need to implement actions of promotion, protection, and support to breastfeeding in the health public services of the municipality.