Medication errors are common and harm hospitalized patients. The authors designed and implemented an automated system to complement an existing computerized order entry system by detecting the administration of excessive doses of medication to adult in-patients with renal insufficiency. Its impact, in combination with feedback to prescribers, was evaluated in 3 participating nursing units and compared with the remainder of a tertiary care academic medical center. The baseline rate of excessive dosing was 23.2% of administered medications requiring adjustment for renal insufficiency given to patients with renal impairment on the participating units and 23.6% in the rest of the hospital. The rate fell to 17.3% with nurse feedback and 16.8% with pharmacist feedback in the participating units (P<.05 for each, relative to baseline). The rates of excessive dosing for the same time periods were 26.1% and 24.8% in the rest of the hospital. Automated detection and routine feedback can reduce the rate of excessive administration of medication in hospitalized adults with renal insufficiency.