OBJECTIVE: To examine the effect of breastfeeding during the postpartum oral glucose tolerance test (OGTT) on maternal blood glucose and insulin among women with recent gestational diabetes mellitus. METHODS: Participants were enrolled in the Study of Women, Infant Feeding, and Type 2 Diabetes, a prospective observational cohort study of 1,035 Kaiser Permanente Northern California members who had been diagnosed with GDM and subsequently underwent a 2-hour 75-g OGTT at 6-9 weeks postpartum for the study enrollment examinations from 2008 to 2011. For this analysis, we selected 835 study participants who reported any intensity of lactation and were observed either breastfeeding their infants (ie, putting the infant to the breast) or not breastfeeding during the OGTT. RESULTS: Of 835 lactating women, 205 (25%) breastfed their infants during the 2-hour 75-g OGTT at 6-9 weeks postpartum. Mean (standard deviation) duration of breastfeeding during the OGTT was 15.3 (8.1) minutes. Compared with not having breastfed during the OGTT, having breastfed during the test was associated with lower adjusted mean (95% confidence interval) 2-hour glucose (mg/dL) by -6.2 (-11.5 to -1.0; P=.02), 2-hour insulin (microunits/mL) by -15.1 (-26.8 to -3.5; P=.01), and natural log 2-hour insulin by -0.15 (-0.25 to -0.06; P<.01), and with higher insulin sensitivity index0,120 by 0.08 (0.02-0.15; P=.02), but no differences in plasma fasting glucose or insulin concentrations. CONCLUSION: Among postpartum women with recent gestational diabetes mellitus, breastfeeding an infant during the 2-hour 75-g OGTT may modestly lower plasma 2-hour glucose (5% lower on average) as well as insulin concentrations in response to ingestion of glucose.