We prospectively examined the causes of medication errors and incidents in a level 3 neonatal intensive care unit for a 2-year period. There were 313 incidents in 23,307 patient days (13.4 incidents per 1000 patient days). The relative risk of incidents was greater in more intensive levels of care. The two most common causes of incidents were neglecting to give a medication on schedule (n = 52) and nonregulation of an intravenous infusion (n = 32). Twenty percent of orders made by physicians that were in error resulted in serious incidents compared with only 6% of all other causes.