Aim: A study was undertaken to examine to what extent psychosocial factors are related to the length of breastfeeding. Methods: A cohort of Danish mothers giving birth to a single child was followed up for four months. Information on mother and baby including psychosocial variables was obtained from a self-report questionnaire. Breastfeeding status was subsequently monitored by a health visitor. Results: A total of 471 (88%) mothers participated, 98.7 % initiated breastfeeding and after four months 277 (59%) were still exclusive breastfeeding; 99 mothers, 51% of those who stopped, stopped within the first five weeks. In Cox regression analyses the duration of breastfeeding showed a positive association with mother's schooling ( p=0.002), her intention to breastfeed ( p=0.001), previous experience with breastfeeding ( p<0.001), self-efficacy with respect to breastfeeding ( p<0.001), her confidence in breastfeeding ( p=0.012) and knowledge about breastfeeding ( p=0.001). The effect of the mother's knowledge depended on the parity of the child. Among primiparous mothers high knowledge was associated with long duration of breastfeeding, but this association was not found among the multiparous. Conclusions: To help the mothers who would like to breastfeed their baby, we must improve our ability to identify mothers at risk of early cessation. Mother's schooling, her intention, self-efficacy and earlier breastfeeding experience can be used as early predictors. An intervention should aim at improving the self-efficacy and resources of these mothers, with a focus on practical knowledge. The first five weeks, when the largest proportion of the cessations occurred, require special attention.