This study describes breastfeeding practices with children born in a university hospital in the city of S o Paulo, Brazil, and identifies factors associated with duration of breastfeeding and exclusive breastfeeding. A cohort of 506 children was identified; of these, it was possible to analyze information on feeding practices for 450 infants at least until the second month of life. Daily information on infant feeding was recorded by mothers in a food frequency questionnaire. Survival analysis techniques (Kaplan-Meier and Cox) were used. Median duration of breastfeeding and exclusive breastfeeding were 205 and 23 days, respectively. The principal factors related to exclusive breastfeeding were mother's age (hr young/old = 1.22; 95% CI = 1.006-1.486) and mother's schooling (hr primary/university = 2.13; 95% CI = 1.381-3.307 and hr secondary/university = 1.78; 95% CI = 1.145-2.792), which could be related to knowledge concerning the benefits of exclusive breastfeeding.