A prospective study of the course of breast feeding was carried out in 75 randomly selected women. Weekly interviews were performed from the day after delivery until the termination of breast feeding, but in no case for longer than 6 months. In each case a detailed analysis was made of the factors leading to transient lactation crises or to complete cessation of breast feeding. A second group of 71 mothers, also randomly selected, was interviewed in retrospect only, 6 months after delivery, and served as controls. Twenty-four weeks after delivery 47% of the mothers in the weekly interview group were still breast feeding. The corresponding figure in the control group was 38%. In both groups only few mothers terminated lactation for medical reasons, while about one fourth stopped for some other reason and about one half because of a combination of factors. Brief case reports are presented to illustrate how varying the factors were that threatened breast feeding.