This study surveyed over 22,800 patients across three health care organizations to assess how often patients who read open ambulatory visit notes perceive mistakes in the notes. The analysis found that 4,830 patients (21%) perceived a mistake in one or more notes in the past 12 months and that 42% of those patients considered the mistake to be somewhat or very serious. The most common very serious mistakes involved incorrect diagnoses; medical history; allergy or medication; or tests, procedures, or results. Older and sicker patients were more likely to report a serious error compared to younger and healthier patients. Using open notes and encouraging patient engagement can improve record accuracy and prevent medical errors