the effects of breast-feeding The effects of breast feeding on the prevalence of Acute Respiratory Infection have been addressed by Brown et al. (1989), who demonstrated fewer days i with Acute Respiratory Infection in breast-fed than in formula fed in fants. However, the reported lower percentage of days ill may reflect a lower number of episodes instead of a shorter duration Our study provides evidence for a lower prevalence of Acute Respiratory Infection as well as a shorter duration of individual episodes in breast-fed infants, by examining the percentage of days ill and the mean duration of individual episodes In this study, none of the participant morhers Smoke ciga rettes. Smoking habits of the fathers were nor investigared Smoking is highly prevalent in the male population of Mexico City. However, in this area cf the city most working males are away from their homes for 12 h/d or longer becausc of thc lengthy commuting time to their work The effect of family size was analyzed using both the number of siblings and the presence of crowding, because each of those Lwu variables affects the morbidity of acute infections differ functioned as effect modifiers. For in- ently. These variables stance, the protective effect of duration of full breast-feeding n incidence of Acute Respiratory Infection was present only for Lhose infants who had siblings, and rhe association of duration breastfeeding with the number of days ill with Acute Respiratory Infection was present only when crowding was removed from the analysis; crowding was nor associated by itself. significantly The mean birth weight, by feeding practice categories, was not different among groups, allowing us to assume that nutri tional status was comparable at base line. Hence, any diffe ence in the morbidity rates cannot be attributed to a nutri tionally impaired immune response of the children(Chandra 1989). Thus, differences in morhidity might be attributed to the feeding mode. The prcscription of antib ctics was infre quent(12% of all events) and unrelated in the analyses to the percentage of days ill Babies shifting from one feeding mode category to the other caused the formula fed group to be a mixture of those who were formula fed since birth and those with a previous history Thus, the risk rates for the formula fed group of breastfeeding. were probahly underestimared, especially in the later months of follow-up