Medication errors are a major concern in the Emergency Department (ED). We examined medication prescribing errors among consecutive adult ED patients during two 10-day periods before and during one 9-day period after implementing computerized provider order entry in an adult ED. 2,073 patients had 5,950, orders. Before (after) implementation there were 3.7 (2.8) potential adverse drug events, 222.0 (21.0) medication prescribing errors, and 5.1 (0) rule violations per 100 orders.