Incidence of breast-feeding has been examined in 2,233 women who gave birth to babies in hospitals in Boston, Brookline, and Newton in August or September, 1963. Information on breast-feeding was obtained from the mothers by mailed questionnaires and by telephone. Twenty-two per cent of the mothers attempted breast-feeding. Five per cent of the total population breast-fed their babies for 6 months or more. Sex and birth weight of babies do not affect frequency of breast-feeding. Social class is found to be the most important variable affecting incidence of breast feeding. Women who are married to students exhibit the highest incidence of breast-feeding (69.3%). Upper social class women breast-feed more frequently (39.8%) than women in lower social classes (13.6%). Students do not show any variation in breast-feeding patterns by age of mother, parity, or birthplace, but these variables do influence the breast-feeding patterns of women in other social classes.