BACKGROUND: The incidence of the neonatal abstinence syndrome, a drug-withdrawal syndrome that most commonly occurs after in utero exposure to opioids, is known to have increased during the past decade. However, recent trends in the incidence of the syndrome and changes in demographic characteristics and hospital treatment of these infants have not been well characterized. METHODS: Using multiple cross-sectional analyses and a deidentified data set, we analyzed data from infants with the neonatal abstinence syndrome from 2004 through 2013 in 299 neonatal intensive care units (NICUs) across the United States. We evaluated trends in incidence and health care utilization and changes in infant and maternal clinical characteristics. RESULTS: Among 674,845 infants admitted to NICUs, we identified 10,327 with the neonatal abstinence syndrome. From 2004 through 2013, the rate of NICU admissions for the neonatal abstinence syndrome increased from 7 cases per 1000 admissions to 27 cases per 1000 admissions; the median length of stay increased from 13 days to 19 days (P<0.001 for both trends). The total percentage of NICU days nationwide that were attributed to the neonatal abstinence syndrome increased from 0.6% to 4.0% (P<0.001 for trend), with eight centers reporting that more than 20% of all NICU days were attributed to the care of these infants in 2013. Infants increasingly received pharmacotherapy (74% in 2004-2005 vs. 87% in 2012-2013, P<0.001 for trend), with morphine the most commonly used drug (49% in 2004 vs. 72% in 2013, P<0.001 for trend). CONCLUSIONS: From 2004 through 2013, the neonatal abstinence syndrome was responsible for a substantial and growing portion of resources dedicated to critically ill neonates in NICUs nationwide.