This study aims to evaluate the situation with breastfeeding in the State of Piauí, Brazil, at the time of the 2006 vaccination campaign. The sample consisted of 1,963 infants (< 1 year of age) from 45 municipalities. Medians and probabilities of breastfeeding categories were calculated by probit analysis. Multivariate logistic analysis was used to analyze the association between independent variables and partial and exclusive breastfeeding. The probabilities were: 16% for exclusive breastfeeding and 18% for predominant breastfeeding at 180 days of age and 58% for breastfeeding at 361 days. Median duration was 67 days for exclusive breastfeeding and 200 days for breastfeeding. The breastfeeding rate was higher for children whose mothers belonged to social classes C, D, and E (p = 0.024) and lived in the countryside (p = 0.009). Not nursing during the first 24 hours of life, using a pacifier, and bottle-feeding reduced the prevalence of breastfeeding (p < 0.001). The prevalence rates are below the recommended standards and reflect the need to prioritize actions to promote breastfeeding in the State.