The influence of lactation centres on breastfeeding patterns, morbidity and nutritional status was assessed through a longitudinal study from birth up to the age of 6 months among 605 mothers and newborns in Guaruja, Sao Paulo, Brazil. Children recruited in the perinatal period who subsequently attended the lactation centres (54%) were exclusively breastfed significantly more at 4 months (43 versus 18%) and at 6 months of age (15 versus 6%), than non-attenders, even after adjusting for confounders. Also, attenders presented less diarrhoea in the last fortnight than non-attenders (10 versus 17%), and their weight for age was significantly better (mean z-scores of 0.26 and 0.02, respectively). Lactation centres are effective in promoting breastfeeding, and their use in areas with short breastfeeding duration should be considered. This is the first report of a significant impact of a breastfeeding promotion programme on children's morbidity and growth.