Breast-feeding and artificial feeding practices in a developing country were examined and related to some social and developmental factors. This was done during the course of a transverse-type growth study of 1,000 infants from birth to 26 months of age. Three types of feeding were identified: breast, mixed, and artificial. The families were divided into three social categories. To give time for the effects of the type of feeding, groups of children at 6, 8, 10, 12, 14, and 16 months of age were studied. In view of the lack of data on feeding effects upon large samples, the study was performed to assess the relative effects of feeding on early childhood development (Bayley Motor and Mental Scales), body length and weight, disease prevalence, clinical signs of allergies, and number of children in the family. Results showed that within the underprivileged group, there was evidence that breast-feeding promoted physical growth, sensory motor development, and resistance to infection. In mothers of both male and female infants who breast-fed, there was a protective effect against subsequent pregnancy up to ten months. Therefore, among the underprivileged in developing countries, every effort should be made to prolong breast-feeding and to assist mothers with health and nutrition education.