A literature review was conducted to identify the drug classes, types of errors, and types of adverse outcomes related to preventable adverse drug events (pADEs). Studies were identified by keyword search of MEDLINE and International Pharmaceutical Abstracts and by a manual search. The search was limited to peer-reviewed literature reporting pADEs in hospitalized patients and the frequencies of at least one pADE characteristic. The frequencies of pADEs and their characteristics were summarized using median and range. Ten studies published between 1994 and 2001 were included in the review. The reported median frequency of pADEs was 1.8% (range, 1.3-7.8%), and the median preventability rate of ADEs in the hospitals was 35.2% (range, 18.7-73.2%). Cardiovascular drugs were implicated for 17.9% of pADEs (range, 4.3-28.1%). Most pADEs occurred in the prescribing stage of the medication-use process and were dose related. Inappropriate prescribing decisions and patient monitoring were the most frequently identified causes of pADEs. The most common adverse outcomes were allergic reactions, hepatic or renal problems, cardiovascular problems, hematologic problems and bleeding, and central nervous system problems. Frequently reported examples of pADEs included antihypertensive overdose associated with bradycardia or hypotension, antiinfectives prescribed despite a history of allergy, warfarin overdose and inappropriate monitoring resulting in hemorrhage, and opioid overdose or underdose associated with respiratory depression or poor pain control, respectively. Despite the heterogeneity of pADEs, the results of this literature review suggest that a few types of drugs, errors, and adverse outcomes constitute a substantial proportion of pADEs. Targeting these high-priority areas could significantly reduce the overall frequency of pADEs.