Factors associated with full breastfeeding (FBF) at 1 week and at 6 months postpartum were examined in a cohort of 856 mother-infant dyads. Questionnaires were mailed at 4 time points over the first 6 months postpartum. At 1 week, 68% of infants were FBF; at 6 months, 23% were FBF. Factors significantly associated with FBF at 1 week were hospital of delivery, residing with a smoker, maternal shiftwork during pregnancy, and having no prior breastfeeding experience. Cox proportional hazards regression analysis showed that residing with a smoker, having consumed caffeine during pregnancy, reporting elevated maternal trait anxiety at 1 week postpartum, having been employed full-time outside the home prior to delivery, and having received anesthesia/analgesia during labor/delivery were associated with earlier cessation of FBF, whereas not having previous breastfeeding experience predicted its continuation. Although most mothers are breastfeeding early on, a number of factors adversely affect its successful continuation. J Hum Lact. 22(3):292–304.