PIP: The feeding literature on the relative risks of diarrhea morbidity to infants on different modes suffers from several methodological problems. 35 studies from 14 countries were reviewed; 83% found that exclusive breastfeeding was protective compared to partial breastfeeding, 88% that exclusive breastfeeding was protective compared to no breastfeeding, and 76% that partial breastfeeding was protective compared to no breastfeeding. When infants receiving no breastmilk are contrasted with infants on exclusive or partial breastfeeding, the median relative risks are 3.0 for those ages 0-2 months, 2.4 for those 3-5 months, and 1.3-1.5 for those 6-11 months. Above 1 year of age, no protective effort of breastfeeding on diarrhea morbidity was evident. When infants receiving no breastmilk are contrasted with those exclusively breastfed, median relative risks are 3.5-4.9 in the 1st 6 months of life. The literature does not suggest that the relative risks of diarrhea morbidity for bottlefed infants are higher in poor families than in those wealthier. The protective efforts of breastfeeding do not appear to continue after breastfeeding cessation. There is evidence of considerably increased diarrhea severity among bottlefed infants. There is a limited, and mostly pre-1950, literature on the relative risks of diarrhea mortality to infants on different feeding modes. 9 studies from 5 countries were reviewed most of which showed that breastfeeding protects substantially against death from diarrhea. When infants receiving no breastmilk are contrasted with those on exclusive breastfeeding, the median relative risk of death from diarrhea during the 1st 6 months of life is 25. When partially and exclusively breastfed infants are contrasted, the median relative risk of death from diarrhea is 8.6. Breastfeeding can be promoted by changes in hospital routine and by giving information and support to mothers. A review of 21 studies from 8 countries shows that, by such promotion, the most likely reductions in the prevalence of nonbreastfed infants are 40% among infants ages 0-2 months, 30% among those 3-5 months, and 10% among those between 6 months-1 year. Theoretical calculations based on these data show that such promotion can reduce diarrhea morbidity rates by 8-20% and diarrhea mortality rates by 24-27% in the 1st 6 months of life. For children ages 0-59 months, diarrhea morbidity rates would be reduced by 1-4% and mortality rates by 8-9%. A recent study in Costa Rica has documented a substantial impact of breastfeeding promotion on neonatal diarrhea morbidity and mortality, and on diarrhea morbidity in infants 0-5 months. The Costa Rican data show good agreement with the theoretical computations presented in this paper. Several important aspects of breastfeeding and diarrhea remain to be clarified by research. However, the need for this research should not delay action to promote breastfeeding and to monitor its effect upon feeding practice and diarrhea. (author's modified)