Duration of breast feeding was studied in 249 randomly chosen, healthy, term infants of Danish origin of which 80.7% participated. Breast feeding was initiated by 99.5% of the mothers. At 3, 6 and 9 months, 71%, 52% and 33%, respectively, were still breast feeding. Only 1 infant (0.5%) was exclusively breast fed beyond 7 months of age. In a Cox multiple regression analysis of factors influencing duration of breast feeding, we found a positive association with maternal education (p < 0.001) and age (p = 0.02) and a negative association with the amount of formula given at the maternity ward (p < 0.001). Six months after delivery, 79% of the mothers with higher school education (> or = 12 years) were still breast feeding, compared to 29% with a low school education (< or = 9 years). There is still a need for an increased effort to support mothers in breast feeding, focusing particularly on younger mothers with short school education. Formula supplements during the first days of life, given to 73% of the infants, were associated with a shorter duration of breast feeding and should be discouraged.