Colostrum protects the newborn from intestinal infection by its content of secretory immunoglobulin A and other immediately acting factors. It may also induce maturation of the child's gastrointestinal immune defences, thus contributing to the protection against diarrhoeal disease later in infancy. To test this hypothesis, a case-control study on breast feeding and diarrhoea was carried out in a periurban community in Guinea-Bissau. The child's age at the start of breast feeding was ascertained soon after birth (n = 279). Subsequent cases of acute diarrhoea (n = 66) were identified at 3-monthly examinations, and four concurrent controls were randomly selected among attendants. Three separate estimates of association showed that the cases tended to have started breast feeding later after birth than the diarrhoea-free controls, but no single test was statistically significant. Early breast feeding might have consequences for diarrhoeal morbidity after the neonatal period.