Frequency, main features and factors associated to introduction of supplemental artificial milk formula at the first month of life were studied in 365 infants born from June 1st, through October 31, 1980, who were under control at a well baby clinic of Santiago, Chile, along years 1986 throughout 1987. By means of life-table analysis, time length of breast feeding was measured and compared in children with and without early formula introduction. In 22.5% of infants an artificial milk formula was given (82/365). In 80% of the cases (66/82) this was done upon instructions of health personnel on grounds of poor weight increase. Early introduction of formula negatively influenced duration of breast feeding which attained a median length of three months in artificially fed infant against ten months in those exclusively breast fed. Main features of infants and mothers prone to early formula introduction were maternal age 35 years or over, previous breast feeding shorter than six months, illegitimacy or first child, fourth or over in birth order, low birth weight and, or, hospital admission in the first four weeks of life. No influence was identified from educational level, of maternal work, desired or unwanted pregnancy, maternal nutritional status in the last three months of pregnancy, child spacing, type of delivery, sex and gestational age of the infants.