OBJECTIVES: We aimed to assess the impact of an automated dispensing system on the incidence of medication errors related to picking, preparation, and administration of drugs in a medical intensive care unit. We also evaluated the clinical significance of such errors and user satisfaction. DESIGN: Preintervention and postintervention study involving a control and an intervention medical intensive care unit. SETTING: Two medical intensive care units in the same department of a 2,000-bed university hospital. PATIENTS: Adult medical intensive care patients. INTERVENTIONS: After a 2-month observation period, we implemented an automated dispensing system in one of the units (study unit) chosen randomly, with the other unit being the control. MEASUREMENTS AND MAIN RESULTS: The overall error rate was expressed as a percentage of total opportunities for error. The severity of errors was classified according to National Coordinating Council for Medication Error Reporting and Prevention categories by an expert committee. User satisfaction was assessed through self-administered questionnaires completed by nurses. A total of 1,476 medications for 115 patients were observed. After automated dispensing system implementation, we observed a reduced percentage of total opportunities for error in the study compared to the control unit (13.5% and 18.6%, respectively; p<.05); however, no significant difference was observed before automated dispensing system implementation (20.4% and 19.3%, respectively; not significant). Before-and-after comparisons in the study unit also showed a significantly reduced percentage of total opportunities for error (20.4% and 13.5%; p<.01). An analysis of detailed opportunities for error showed a significant impact of the automated dispensing system in reducing preparation errors (p<.05). Most errors caused no harm (National Coordinating Council for Medication Error Reporting and Prevention category C). The automated dispensing system did not reduce errors causing harm. Finally, the mean for working conditions improved from 1.0±0.8 to 2.5±0.8 on the four-point Likert scale. CONCLUSIONS: The implementation of an automated dispensing system reduced overall medication errors related to picking, preparation, and administration of drugs in the intensive care unit. Furthermore, most nurses favored the new drug dispensation organization.