A cohort study of healthy term infants was conducted to identify factors associated with breastfeeding duration. Three hundred and fifty mothers delivering in one obstetric hospital in Auckland, New Zealand were recruited. Ninety-five percent were followed up until 1 y. Breastfeeding was initiated by 97.4%, the median duration of breastfeeding was 7.6 mo, and 30% were continuing some breastfeeding at 12 mo. Adjusted risk ratios for shorter duration of breastfeeding were: maternal age <25 y, 2.33 (95% confidence interval = 1.33, 4.05); maternal age 25-34, 1.45 (1.01, 2.09) compared to maternal age > or =35; planning to cease breastfeeding < or =6 mo, 2.39 (1.65, 3.46); planning to breastfeed for as long as possible, 1.48 (1.00, 2.18), or not knowing plans 2.13 (1.36, 3.32) when compared to planning to cease breastfeeding after 6 mo; inverted nipples, 2.02 (1.26, 3.23), daily dummy use 1.62 (1.20, 2.18) or use of formula in the first month 2.79 (2.05, 3.80). Reporting mastitis, 0.67 (0.48, 0.94), and sharing the mother's bedroom at 3 mo, 0.69 (0.51, 0.92) were associated with a reduced risk for shorter duration of breastfeeding. We recommend that during the antenatal period attention be directed at mothers' plans for duration of breastfeeding, that mothers be encouraged to have their baby in their bedroom, and that the use of formula and dummies should be discouraged in the first months of life.