High prepregnant BMI is associated with reduced initiation and duration of breast-feeding (BF). To examine how gestational weight gain (GWG) might modify this association, over a 9-y period, we identified all women (n = 2783) who had attempted to breast-feed their newborns. From their medical records, we categorized them by prepregnant BMI [as underweight (<19.8 kg/m2), normal-weight (19.8-26.0 kg/m2), overweight (26.1-29.0 kg/m2) or obese ( >29.0 kg/m2)] and GWG [as below, within, or above the amount recommended by the Institute of Medicine]. Women with a normal BMI who gained within these recommendations served as the reference group in regression analyses, which were adjusted for confounding factors. Both normal-weight (P < 0.05) and obese (P < 0.01) women who exceeded the recommended GWG had higher odds of failing to initiate BF (defined as continuing to breast-feed at 4 d postpartum). Underweight (P < 0.05), overweight (P < 0.05), and obese (P < 0.01) women who exceeded the recommendations for GWG as well as obese women who gained within the recommendations (P < 0.01) had a higher risk of early discontinuation of exclusive BF. Only obese women who gained within or exceeded the recommendations (P < 0.01) for GWG had a higher risk of early discontinuation of any BF. Excessive GWG was associated with a measure of failure to initiate and/or sustain BF in all categories of prepregnant BMI. Thus, in addition to conceiving at a healthy weight, gaining the recommended amount of weight during pregnancy is also important for successful BF.