In a prospective study on breast-feeding in Jerusalem, 274 middle-class Jewish women were interviewed about their breast-feeding practices, and symptoms and signs of disease, episodes of illness and hospitalization of the infant. Women of a higher education level breast-fed more often and for a longer period than did women with less education. Infants exclusively breast-fed had significantly fewer symptoms of disease than did those not breast-fed or partially breast-fed. The odds ratios for cough, respiratory difficulty, and diarrhea by breast-feeding practice were 3.66, 2.14 and 2.72 (P = 0.04). Significant differences in the number of illness episodes were found between breast-fed and bottle-fed infants at 20 weeks; infants exclusively breast-fed had the least number of illness episodes. A positive association was found between number of illness episodes and duration of breast-feeding. Infants who were breast-fed for 20 weeks had the least number of illness episodes; 52% of them had no episode compared with only 15% who were not breast-fed. Comparison of the numbers of illness episodes among non-breast-fed infants of mothers with low and high education levels indicated that the infants of better educated mothers had a significantly lower percentage of illness episodes (P less than 0.05). Even infants of a middle-class and well-educated population benefit from the breast-feeding practice and its protective effect, more so if they are exclusively breast-fed and for a longer period.