Although most care occurs in outpatient settings, research into adverse events (AE) in this setting remains sparse in comparison to acute care. In this study, the medical records of patients who received outpatient care (e.g., primary, radiology, emergency, ambulatory surgery) in Massachusetts in 2018, were reviewed for any occurrence of adverse events. Seven percent had at least one AE, most commonly adverse drug events, and 23% were potentially preventable. Most AE originated from care in the physician's office.