BACKGROUND AND AIM[|]Per- and polyfluoroalkyl substances (PFAS) are widely detected in pregnant persons and can be transferred to the developing fetus in utero. After birth, infants may continue to be exposed to PFAS via breastfeeding. However, studies quantifying levels of PFAS in breastmilk samples remain scarce, particularly in low- and middle-income countries. Our objective was to examine breastmilk as a postnatal PFAS exposure source among mother-infant pairs in Thailand.[¤]METHOD[|]Pregnant farmworkers were enrolled in the Study of Asian Women and their Offspring’s Development and Environmental Exposures (SAWASDEE), a prospective birth cohort in Northern Thailand, between 2017 and 2019. We quantified levels of eight PFAS in maternal serum samples obtained during the second trimester, as well as in breastmilk samples obtained at 9.5 months and 11.5 months in infancy (N=46 matched pairs). For each PFAS, we calculated lactational (blood to milk) transfer efficiencies and lactational estimated daily intake.[¤]RESULTS[|]The average maternal age in our study population was 25.7 years (standard deviation [SD]=5.81) and 95.6% of participants were married. PFOA, PFOS PFHxS, PFNA, PFDA, and PFUnDA were detected in >90% of serum samples. PFOS was detected in >60% of breastmilk samples obtained at 9.5 and 11.5 months, while PFNA was detected in >50% of 11.5-month breastmilk samples only. All remaining PFAS were detected in <50% of breastmilk samples. The breastfeeding transfer efficiency for PFOS was 7.03% (SD=5.78) and 5.83% (SD=5.21) at 9.5 and 11.5 months, respectively. The lactational estimated daily intake for PFOS was 12.1 ng/kg bodyweight/day (SD=5.49) and 10 ng/kg bodyweight/day (SD=6.22) at 9.5 and 11.5 months, respectively.[¤]CONCLUSIONS[|]Among pregnant farmworkers in Thailand, we found that legacy PFAS were widely detected in serum, and some compounds were also detected in breastmilk. This study provides new evidence enhancing our understanding of postnatal exposure to PFAS.[¤]