This retrospective cohort study conducted reviews of both inpatient and outpatient medical records to determine the incidence and types of errors identified. Using a random sample of more than 600 patients from a Veterans Affairs health care system, investigators discovered nearly 3000 errors. The large majority of errors were attributed to the underuse of care, particularly in patients suffering from chronic diseases. The authors conclude that safety efforts and ongoing quality improvement should focus as much on errors of omission as they do on errors of commission.