This study was a comparative analysis of factors affecting duration of breastfeeding among users of the São Marcos primary care clinic in Belo Horizonte, Minas Gerais State, Brazil, in 1980, 1986, 1992, 1998, and 2004. Five retrospective longitudinal studies (historical cohorts) were performed with the same questionnaire, and 790 mothers of children less than 24 months of age were interviewed. The statistical analysis was conducted year-by-year using the Kaplan-Meier method and Cox model. From 1980 to 2004, conditions significantly associated with risk of weaning were: primiparity; difficulty in postpartum breastfeeding; belief in ideal breastfeeding duration of less than six months; start of breastfeeding after discharge from the maternity hospital, non-recognition of the advantages of breastfeeding for the child; and unfavorable, unknown, or indifferent paternal opinion concerning breastfeeding. In four of the five studies, difficulty in breastfeeding (RR: 1.70-3.97) and belief in ideal breastfeeding duration of less than six months (RR: 1.67-3.27) were risk factors for weaning. Median duration of breastfeeding was five months in 1980 and 11 months in 2004.