Exclusive breastfeeding is promoted by the World Health Organization (WHO) with the aim of improving infant health. The specific objective of the study was to examine the effect of exclusive breast-feeding on morbidity and mortality in a randomised prospective study. Children born by mothers recorded as living in the study area during pregnancy was randomised at birth. WHO recommendations to postpone introduction of water and weaning food were told to the mother by 2-weekly home visits. All study children were followed from birth till 1 year of age according to morbidity, hospitalisation and mortality.