Objective: To evaluate factors that can influence the Finnegan Neonatal Abstinence Score (FNAS). Study design: Retrospective analysis of 367 patients admitted to two level IV neonatal intensive care units. Linear mixed effects models were developed to evaluate daily census, time of the day, and day of the week as fixed effect predictors. The degree of influence that nurses had on FNAS variability was also estimated. Results: Bivariate analyses showed that daily census and the time of day have significant influence on the FNAS in institution 1, with minimal clinical significance. The proportion of variation in the FNAS attributable to differences in nurses was of 9.8% and 5.1% for institutions 1 and 2, respectively (P<0.0001). Conclusions: The minimal influences of extraneous factors on the FNAS support the clinical utility of the scoring system in the assessment and management of infants with Neonatal Abstinence Score.