A study of breast-feeding practices over the first 6 months of life among a cohort of urban poor infants in southern Brazil indicated that the median duration of breast-feeding was 18 weeks, and at 6 months 41% of the infants were still being breast-fed. The duration of breast-feeding was significantly associated with the following: the infant's sex, mother's colour, type of first feed, timing of the first breast-feed, breast-feeding regimen and frequency of breast-feeding at 1 month, and the use of hormonal contraceptives by the mother. The following were significant risk factors for early termination of breast-feeding: the infant's sex, type of first feed, use of supplementary feeds, frequency of breast-feeding, feeding regimen, weight-for-age, and weight-for-age after controlling for birth weight. Dissatisfaction with their infant's growth rate was the most frequent reason given by mothers for supplementing the diets of infants who were exclusively breast-fed in the first 3 months of life. Also, the mothers' perception that their milk output was inadequate was the most frequent reason expressed for stopping breast-feeding in the first 4 months. The roles of health services and family support in providing favourable conditions for increasing the duration of breast-feeding in the study population are discussed, as well as the possibility of bias being introduced into studies of the relationship between infant feeding and growth by the effect of the infant's rate of growth on the mother's decision to continue breast-feeding.