Medication errors are common in neonatal intensive care unit (NICU) settings. This study used data from MEDMARX, a voluntary reporting system for medication errors, to analyze the underlying causes of adverse drug events in the NICU. While most errors did not cause patient harm, prescribing errors, errors involving malfunctioning equipment, and errors associated with known high-risk medications were more likely to cause clinical consequences. Most errors were ascribed to human factors causes. While prior research has found that computerized provider order entry (CPOE) can reduce medication errors in the NICU, it is notable that nearly half the reported errors in this study occurred during medication administration, and therefore would not have been prevented by CPOE.