OBJECTIVE Evaluation of a computerized physician order entry in an Internal Medicine Department, with a unit-dose distribution system. SETTING Pharmacy Department, Internal Medicine Department. S. Francisco Xavier Hospital, Lisbon, Portugal. METHOD This study was carried out in December 2001 and January 2002. After two years experience of the CPOE system, medication errors were evaluated prospectively, in an internal medical department of a 360-bed academic hospital. Data were collected once a week. Pharmacists reviewed all medical prescriptions as part of their routine work. Medication errors detected were recorded on a data collection form with a design based on the types of errors as defined by the American Society of Hospital Pharmacists (ASHP). Completed forms were reviewed and medication errors were classed according to ASHP guidelines. RESULTS A total of 2268 orders were monitored (162 patients). In these orders, 73 medication errors (22.4% of the patients) were detected and documented (59 prescribing errors and 14 monitoring errors). The most common prescribing errors were deficiencies related to the right class but wrong drug (28.3%): omeprazole vs. ranitidine/sucralfate in stress ulcer prophylaxis; incorrect dose (30%) and unclear orders (13.3%). Errors related to incorrect frequency of administration (5%); maintenance of IV route (5%); duplicated drug therapy (11.7%); drug interactions (1.7%) and length of therapy (3.3%) were also detected. The 14 monitoring errors detected were failures to review a prescribed regimen for appropriateness and detection of problems. CONCLUSIONS Computerized prescription order entry has demonstrated effectiveness in eliminating medication errors related to transcribing and patient identification. Nevertheless, medication errors related to prescription and monitoring still occur. The use of clinical decision support systems and pharmacist involvement is vital to achieve maximum medication safety and reduce medication error rates.