OBJECTIVE . To quantify the type and frequency of drug administration errors to pediatric in-patients and to identify associated factors. DESIGN Prospective direct-observation study of drug administration errors from April 2002 to March 2003. SETTING Four clinical units in a pediatric teaching hospital. STUDY PARTICIPANTS Twelve observers accompanied nurses giving medications and witnessed the preparation and administration of all drugs to all patients on all weekday mornings. INTERVENTION None. MAIN OUTCOME MEASURE Discrepancies between physicians' orders and actual drug administration. RESULTS During the 1719 observed administrations to 336 patients by 485 nurses, 538 administration errors were detected, involving timing (36%), route (19%), dosage (15%), unordered drug (10%), or form (8% form). These errors occurred for 467 (27%) of the 1719 administrations. Intravenous drugs (OR = 0.28; CI = 0.16-0.49; versus miscellaneous) were associated with fewer errors. Error rates were higher for cardiovascular (OR = 3.38; CI = 1.24-9.27; versus miscellaneous) and central nervous system drugs (OR = 2.65; CI = 1.06-6.59; versus miscellaneous); unspecified dispensing system (OR = 2.06; CI = 1.29-3.29; versus store in the unit); non-intravenous non-oral administration (OR = 4.44; CI = 1.81-10.88; versus oral administration); preparation by the pharmacy (OR = 1.66; CI = 1.10-2.51); and administration by a hospital pool nurse, temporary staffing agency nurse, or nurse intern (OR = 1.67; CI = 1.04-2.68; versus registered full-time nurse). Each additional management procedure in the patient increased the risk of error (OR = 1.22; CI = 1.01-1.48). CONCLUSIONS The risk factors identified in our study should prove useful for designing preventive strategies, thereby improving the quality of care.