The main objectives of the present study were to understand the knowledge and practice of breastfeeding mothers about the feeding and supplementary food and to analyze the factors related to it. The data for the present study collected from July 9 to July 31, 1984 for 302 mothers in Seoul. 1. The Knowledge of Breastfeeding Mothers about the Fee4ing and Supplementary Food (1) In the knowledge of mothers on the appropriate feeding pattern before 6 month, 69.3% of total women believed that breast-fed babies are healthier. There was a singificant difference in the type of feeding in order to education level, socio-economic state, and. mass-media exposure. (2) In the knowledge of mothers on an appropriate duration of lactation, the mean duration of breastfeeding is 8. 1 month. The duration of breastfeeding gradually decreases in order of education level, socio-economic state, mass-media exposure, and employment status. (3) In the knowledge of mothers on an appropriate time of introduction of supplementary food, the mean time of introduction of supplementary food is 4. 7 month. It is gradually decrease in order of education level, socioeconomic state, mass-media exposure, and employment state. 2. The Practice of Breastfeeding Mothers about the Feeding and Supplementary Food (1) The practice of mothers on infant-feeding before 6 month and that education level, socio-economic state and mass media exposure are the higher, the rate of breast feeding is the less. (2) The mean duration of breastfeeding is 9 month. It is 1 month longer the knowledge of mothers on an appropriate feeding pattern before 6 month. (3) The time of introduction of supplementary food is average 4.75 month. It is similar to the knowledge of mothers on an appropriate time about introduction of supplementary food. The result of this study indicate that the knowledge and practice of breast feeding is maintained comparatively high in low educated group, whereas it is noticeably decreasing among the high educated group. High educated group believed that the short duration of breastfeeding better than long, and they practiced breastfeeding so short period. And high educated group also thought early introduction of supplementary food is good for the babies health. Consequently, in this study, for the most part, elite group is found to have rather wrong opinions and practice about infant nutrition. Therefore the accurate information of infant nutrition is must be encouraged to mothers.