BACKGROUND A clear understanding of patients' understanding and perceived risk of medical errors is needed. Multiwave telephone interviews were conducted in 2002 with 1,656 inpatients from 12 Midwestern hospitals regarding patients' conceptualization of medical errors and perceived risk of seven types of medical errors. RESULTS Patients defined medical errors to include not only clinical mistakes but also falls, communication problems, and responsiveness. Ninety-four percent of respondents reported their medical safety as good, very good, or excellent, but 39% experienced at least one error-related concern, most commonly medication errors (17% of respondents), nursing mistakes (15%), and problems with medical equipment (10%). Frequency of concerns was associated with reduced willingness to recommend the hospital (p < .001). DISCUSSION If patients' definition of medical errors is broader than the traditional medical definition, providers should clarify the term "error" to ensure effective communication. Most patients felt a high level of medical safety but a sizeable proportion experienced a concern about an error during hospitalization. The selective nature of concerns and the impact of patient and hospital characteristics provide insight into ways to engage patients in error prevention programs.