An ambitious breastfeeding promotion programme was launched in Brazil in 1981 which included the use of mass media, health personnel training and re-organization of health service routines. A comparison of two health surveys carried out in Greater São Paulo before (1981) and after (1987) the implementation of the programme showed an important increase in the median duration of breastfeeding from 84 to 146 days, respectively. The potential impact of the programme on infant mortality rates has been estimated, using the relative risks associated with breastfeeding obtained in a case-control study in a similar urban population in southern Brazil. During this period the infant mortality rate in São Paulo fell by 49%. The observed change in feeding practices may have been responsible for a reduction of 12% of this 49% or, expressed another way, for one-quarter of the observed decline in infant mortality. The calculations suggest that the programme may have led to reductions in deaths caused by diarrhoea of 32%, in respiratory infections of 22% and in deaths due to other infections of 17%.