This study examined nurses reasons for medication errors, reasons for not reporting errors, and perceived unit-reporting practices. It compared nurses anonymous reports of medication errors with those from institutional reporting mechanisms. Qualities of the work environment, staffing, and workload were evaluated to determine associations with perceived error-reporting practices. The study findings have immediate applicability as a baseline for system improvements. © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins.