Evaluation studies of Computerized Physician Medication Order Entry (CPOE) systems are important for assessing their often stated beneficial effects on health care. This paper provides a systematic literature review of articles published from 1950 to 2006 that evaluate one of six aspects of CPOE in the outpatient setting: safety; cost and efficiency; adherence to guideline; alerts; time; and satisfaction, usage, and usability. Thirty articles with original data and randomized clinical trial, non-randomized clinical trial, or observational study designs met the inclusion criteria. Only four studies assessed the effect of CPOE on safety. The effect was not significant on the number of adverse drug events (ADEs); and in one study the effect on number of medication errors was significant. Three studies showed that CPOE significantly reduced the cost of prescriptions; five other studies could not demonstrate a significant cost reduction. Most studies on adherence to guidelines showed a significant positive effect. More than half of alerts were ignored by physicians. The three studies focusing on time for the direct and indirect patient care showed it increased significantly. In spite of the cited potential merits of CPOE systems for enhancing safety and reducing costs, the relatively small number of evaluation studies published to date do not provide adequate evidence that CPOE systems provide these benefits in outpatient settings. There is however evidence for (a) increasing adherence to guidelines, (b) increasing total prescribing time, and (c) high frequency of ignored alerts. Evaluating the impact of CPOE in outpatient settings is complicated by incomplete and scattered patient data and the difficulty of carrying out rigorous controlled studies in such settings.