'Na prática, a teoria è outra' (in practice, the theory is different) is an old Brazilian saying. This phrase summarizes well the general practice of breastfeeding in Brazil: 'Breast is best' is central in the pregnant women's future oriented 'theory' of how their infant should be fed. In the subsequent weeks after delivery, however, in the daily practicalities of feeding their infant, this theory is, to a large extent, abandoned. The present study is based on a sample of 300 mothers in the city of Aracaju in the Northeast of Brazil. Through interviews, the differences and similarities between knowledge and practice with respect to infant feeding were established. An explanation of these differences is developed on the basis of a processual analysis of the qualitative and quantitative results of the interview data. Nearly all mothers were knowledgeable of the need to breastfeed, and nearly all mothers had initiated breastfeeding. However, only a minority was exclusively breastfeeding at the time of the interview. A distinction is made between a breastfeeding process and a de-breastfeeding process. The data suggest that mothers, in general, start the de-breastfeeding process with the positive intention of ameliorating the infant's situation without realizing the negative processual consequences that most likely ends in a cessation of breastfeeding. The study supports the view that health policy should underline the processual character of both breastfeeding and de-breastfeeding when promoting the importance of exclusive breastfeeding.