More than a decade after doctors began using tiny balloons to reopen clogged coronary arteries, the long-term success of the procedure has yet to live up to its potential, heart experts say.

Despite its widespread use - more than 750,000 balloon angioplasties were performed last year worldwide, about 300,000 of them in the United States - nearly one in every three patients undergoing the procedure has to return for another attempt or go through coronary bypass surgery within three to six months.Molecular biologists studying what goes on inside the arteries are only now beginning to understand what happens in these patients: For angioplasty to work, the tiny balloon must tear and crack at the cholesterol and plaque deposits on the inner wall of the artery that block the supply of oxygen-rich blood to the heart muscle. But almost as soon as that is accomplished, the blood vessel begins a sustained effort to heal itself, sending cells from within the lining of the wall that secrete substances aimed at repairing the damage.

Instead of cholesterol, this time the wall becomes clogged with proliferating cells that swell up and block the flow of blood through the artery at the same site that was cut off before the procedure.

"What we are getting in some of these patients is a kind of scar tissue in the artery. It creates an unusual paradox for us: The better we are at reopening the vessel, the more likely we are to cause restenosis," the name given to arteries that clog again after surgery or angioplasty, said Dr. W. Michael Kavanaugh, an adjunct professor of medicine at the University of California, San Francisco. "Until we can intervene to change that, our success at angioplasty will be compromised."

Improving the success of angioplasty has important financial implications for not just patients, but the nation's health-care system as well.

Angioplasty was once hoped to be a more cost-effective procedure than coronary bypass surgery.