Background: Maternal obesity is a risk factor for childhood obesity; this is a major public health concern given that similar to 40% of pregnant women are either overweight or obese. Whether differences in milk composition in lean compared with obese women contribute to childhood obesity is unclear. Objectives: We aimed to analyze relationships between maternal obesity and human milk metabolites, infant body composition, and postnatal weight gain. Methods: This was a prospective study in which mothers intending to breastfeed exclusively, and their newborn infants, were enrolled at delivery (n = 35 mother-infant pairs). We excluded mothers with diabetes, other medical conditions, or pregnancy complications. Participants were grouped by maternal prepregnancy BMI < 25 (lean) or >= 25 kg/m(2) (overweight/obese). We analyzed infant body composition by dual-energy X-ray absorptiometry and used untargeted liquid chromatography-gas chromatography-mass spectrometry to measure the milk content of 275 metabolites at 1 and 6 mo postpartum. Results: At 1 mo postpartum, 10 metabolites differed between overweight/obese and lean groups with nominal P < 0.05, but none was altered with a false discovery rate < 0.25. Many differentially abundant metabolites belonged to the same chemical class; e.g., 4/10 metabolites were nucleotide derivatives, and 3/10 were human milk oligosaccharides. Milk adenine correlated positively with both continuously distributed maternal BMI and with infant adiposity and fat accrual. Analysis of milk composition at 6 mo postpartum revealed 20 differentially abundant metabolites (P < 0.05) in overweight/obese compared with lean women, including 6 metabolites with a false discovery rate of < 0.25. At both 1 and 6 mo, human milk abundance of 1,5-anhydroglucitol, which has not previously been described in milk, was positively associated with maternal BMI. Conclusions: Maternal obesity is associated with changes in the human milk metabolome. While only a subset of metabolites correlated with both maternal and infant weight, these point to potentialmilk-dependent mechanisms for mother-child transmission of obesity.