A longitudinal prospective study was performed to investigate the total duration of breast-feeding. Three hundred and sixty six mothers were followed-up until three years after delivery. At three months, 73% were breast-feeding, at six months 52%, at one year 20% and 1% breast-fed more than three years. Correlations were found between duration of breast-feeding and maternal age, education and social class. Mature mothers who lived under good socioeconomic conditions breast-fed for longer periods than those who were less fortunate in these respects. Mothers who had breast-fed an older child usually breast-fed the next child for a similar period. Breast-feeding has become popular: only 1% did not want to breast-feed at all and 89% of mothers intended to breast-feed their children for six months or more. Solid food was most frequently introduced when the infants were four months old. At six months 4% were still exclusively breast-fed. Even though the mothers now breast-feed longer than previously, only 39% succeeded in breast-feeding for as long as or longer than they had intended.