Objective: This study aims to explore the effect of feeding type (breast milk, formula, or both) on the length of stay (LOS), requirement of replacement therapy, and relapse rates in infants diagnosed with Neonatal Abstinence Syndrome (NAS). Methods: We conducted a retrospective chart review of 200 neonates admitted to a Level III Neonatal Intensive Care Unit with a diagnosis of NAS without comorbidity between August 2012 and August 2014. Approval for this study was obtained from the Institutional Review Board (IRB) at East Tennessee State University. During each infant’s hospitalization, an institutionally-developed objective scoring tool was used to assess signs of NAS every 4 hours, and neonates were treated with opiate replacement therapy for two consecutive scores >10. Patients were discharged when they had no increased scores for 48 hours after the morphine was discontinued. The information collected included gestational age, feeding type, LOS (measured in days), and requirement of opiate replacement therapy as well as information about recurrence of NAS signs after weaning of replacement therapy or treatment discontinuation. T-test, F-test, chi-square test, and fisher test were used to compare the outcomes of the subgroups. Results: In the study population, 22 babies (11%) were fed exclusively breast milk (BMB), 122 (61%) were fed exclusively formula (FFB), and 56 (28%) were fed both breast milk and formula (BFB). The LOS was significantly longer (p < 0.0001) in FFB (mean 20.4 days) than in either BMB (10.6 days) or BFB (13.8 days) regardless of gestational age (Figure 1). FFB were significantly more likely (p=0.0004) to require opiate replacement (90.2%) than BMB (59.1%) and BFB (75.0%); FFB were also significantly more likely (p=0.0153) to experience relapse of NAS signs after discontinuation of opiate replacement and during the weaning process (30.3%) than BMB (9.1%) and BFB (14.3%) (Figure 2). Conclusion: In this study, infants diagnosed with NAS who were fed exclusively formula had significantly increased length of hospital stay, increased requirement for opiate replacement therapy, and increased episodes of failed opiate replacement weaning or relapse of NAS symptoms when compared with infants fed exclusively breast milk or a combination of breast milk and formula. Feeding with breast milk should be recommended to all mothers of infants diagnosed with neonatal abstinence syndrome to decrease the infants’ duration of stay, symptom severity, and risk for relapse.