It is estimated that only one out of seven errors, accidents and other events are recognized and reported. That means they can never be tracked or corrected.
The study is from the inspector general of the Department of Health and Human Services.
The New York Times says federal investigators identified many unreported events by having independent doctors review patients' records.
Those harmful mistakes include hospital-acquired infections, delirium from overuse of pain fillers, severe bedsores and excessive bleeding.