Avoiding use of unclear or misleading abbreviations is a key step in preventing medication prescribing errors, and the Joint Commission mandates avoiding specific abbreviations as one of its National Patient Safety Goals. This study analyzed Medmarx data from 2004 to 2006 to determine the prevalence and impact of errors related to abbreviations. Despite dissemination of the Joint Commission's “do not use” abbreviation list, errors involving these abbreviations occurred more than 18,000 times during the study period, although few patients were harmed as a result.