The breast-milk composition in the first 6 wk postpartum of women who have undergone bariatric surgery (BS) is unknown. The aim of this study was to examine 1) the breast-milk macronutrient and vitamin A composition in women who had and who had not undergone BS and 2) the impact of maternal diet on the breast-milk composition. We hypothesized that the milk of women who had undergone BS would be less energy dense and have a lower vitamin A concentration than that of other women. A multicenter prospective substudy was conducted at 2 university hospitals. Breast-milk samples were collected from 24 normal-weight [NW; mean ± SD body mass index (BMI; kg/m2): 21.5 ± 1.7; mean ± SD age: 29 ± 6 y], 39 overweight (OW; BMI: 26.9 ± 1.5; aged 29 ± 5 y), and 12 obese women (BMI: 35.0 ± 5.7; aged 29 ± 5 y) as well as from 11 women who had undergone BS (BMI: 28.0 ± 4.4; aged 30 ± 4 y) from day 3 until week 6 of lactation. Milk energy and macronutrients (Human Milk Analyzer; Miris) and vitamin A concentrations (iCheck Fluoro; BioAnalyt) were determined at the end of each week. Maternal diet (food-frequency questionnaire) and physical activity (Kaiser Physical Activity Survey) were measured during the third trimester of pregnancy and on day 3 or 4 and during week 6 of lactation. Statistical analyses include 1-factor ANOVA, Spearman and Pearson correlations, and multiple linear regression. In all women, a weekly increase in milk energy, total fat, and total carbohydrates was seen, whereas a weekly decrease in proteins and vitamin A was found during the first 2 wk of lactation, followed by a stable concentration of all nutrients. At week 4, milk protein concentrations were higher in women who had undergone BS (14 g/L) compared with NW (8 g/L; P = 0.005) and OW (9 g/L; P = 0.019) women. At week 5, milk carbohydrate concentrations were higher in women who had undergone BS (74 g/L) compared with NW women (68 g/L; P = 0.042). Breast milk of women who have undergone BS appears to be adequate in energy, macronutrients, and vitamin A during the first 6 wk of lactation. This supports the conclusion that breast feeding should not be discouraged in this group of women. This trial was registered at http://www.clinicaltrials.gov as NCT02515214.