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Neonatal abstinence syndrome (NAS) due to in-utero opioid exposure has significant variability of severity. Preliminary studies have suggested that epigenetic variation within the μ-opioid receptor (OPRM1) gene impacts NAS. We aimed to determine if DNA methylation in OPRM1 within opioid-exposed mother-infant dyads is associated with differences in NAS severity in an independent cohort. Full-term opioid-exposed newborns and their mothers (N = 68 pairs) were studied. A DNA sample was obtained and then assessed for level of DNA methylation at 20 CpG sites within the OPRM1 promoter region by next-generation sequencing. Infants were monitored for NAS and treated with replacement opioids according to institutional protocol. The association between DNA methylation level at each CpG site with NAS outcome measures was evaluated using linear and logistic regression models. Higher methylation levels within the infants at the -18 (11.4% vs 4.4%, P = .0001), -14 (46.1% vs 24.0%, P = .002) and +23 (26.3% vs 12.9%, P = .008) CpG sites were associated with higher rates of infant pharmacologic treatment. Higher levels of methylation within the mothers at the -169 (R = 0.43, P = .008), -152 (R = 0.40, P = .002) and +84 (R = 0.44, P = .006) sites were associated point-wise with longer infant length of stay. Maternal associations remained significant point-wise for -169 (β = 0.07, P = .007) and on an experiment-wise level for +84 (β = -0.10, P = .003) using regression models. These results suggest an association of higher levels of OPRM1 methylation at specific CpG sites and increased NAS severity, replicating prior findings. These findings have important implications for personalized treatment regimens for infants at high risk for severe NAS.