DOCTORS make mistakes. They may be mistakes of technique, judgment, ignorance or even, sometimes, recklessness. Regardless of the cause, each time a mistake happens, a patient may suffer. We fail to uphold our profession’s basic oath: “First, do no harm.”

According to a 1999 report by the Institute of Medicine, as many as 98,000 Americans were dying every year because of medical mistakes. Today, exact figures are hard to come by because states don’t abide by the same reporting guidelines, and few cases gain as much attention as that of Rory Staunton, the 12-year-old boy who died of septic shock this spring after being sent home from a New York hospital. But a reasonable estimate is that medical mistakes now kill around 200,000 Americans every year. That would make them one of the leading causes of death in the United States. Why have these mistakes been so hard to prevent?

Here’s one theory. It is a given that American doctors perform a staggering number of tests and procedures, far more than in other industrialized nations, and far more than we used to. Since 1996, the percentage of doctor visits leading to at least five drugs’ being prescribed has nearly tripled, and the number of M.R.I. scans quadrupled.

Certainly many procedures, tests and prescriptions are based on legitimate need. But many are not. In a recent anonymous survey, orthopedic surgeons said 24 percent of the tests they ordered were medically unnecessary. This kind of treatment is a form of defensive medicine, meant less to protect the patient than to protect the doctor or hospital against potential lawsuits.