The purpose of this study was to assess the factors associated with exclusive breastfeeding (EBF) during the first 6 months of life. A prospective, cohort study was conducted in 291 newborn children in 3 semirural localities in Mexico. Data were collected on infant-feeding practices, anthropometry, morbidity, and maternal and household characteristics. Two-stage multivariate regression models were used for statistical analysis. Prevalence of EBF was 50% at day 5. Average duration of EBF was 2.18 months. According to the 2-stage model, maternal age, maternal work, and better socioeconomic conditions were associated with EBF at day 5. Infant nutritional status, prevalence of diarrhea, giving colostrum to the newborn, and maternal experience in breastfeeding were associated with duration of EBF. The prevalence of EBF and its duration were much shorter than recommended internationally. EBF during the early days of life and EBF duration are 2 different processes that are determined by different factors.