ATLANTA (AP) _ A single, rare inherited heart defect is to blame in about half of all cases in which athletes drop dead on basketball courts and playing fields, a study concludes.

While cocaine occasionally triggers fatal heart problems in athletes, doctors say unrecognized congenital problems are far more common causes.

Though uncommon, such sudden deaths often get wide attention, especially when they strike star athletes, such as basketball players Hank Gathers of Loyola Marymount University in Los Angeles, who died in 1990, and Reggie Lewis of the Boston Celtics, who died this year.

Dr. Barry J. Maron of the Minneapolis Heart Institute surveyed nearly 100 such deaths and found that a condition called hypertrophic cardiomyopathy underlies about 50 percent of cases. This inherited condition involves a thickening of the wall of the heart's main pumping chamber.

However, this is not the only hazard to well-conditioned athletes. Maron's study found that about 10 other inherited heart abnormalities can cause sudden death during exercise.

Maron presented his findings Monday at the annual scientific meeting of the American Heart Association. He analyzed the deaths of athletes between the ages of 13 and 40, nearly all of whom died during or soon after competition or practice. Most played basketball or football, and 90 percent were male.

The second most common cause of death in these athletes was inborn defects in the arteries that supply blood to the heart. They accounted for about 15 percent of the cases studied. Such abnormalities can suddenly cut off blood to the heart, resulting in a heart attack.

Another occasional cause of sudden death in athletes was myocarditis, a scarring of heart muscle that results from inflammation. It is caused by viral infections and other problems. This is probably what killed both Lewis and Gathers.

Maron said these three conditions are often considered to be ominous enough to disqualify athletes from competition. However, sometimes the problems are not discovered until an athlete collapses.

Other potential causes of athletes' deaths include Marfan syndrome, heart valve deformities and clogging of the heart arteries.

Dr. William Strong of the Medical College of Georgia said doing a physical and taking a family history can often turn up these problems before youngsters join athletic programs. A history of fainting during exertion is frequently a clue to heart abnormalities.

Dr. Richard Kerber of the University of Iowa said emergency crews who stand by during games should be trained in using a defibrillator, a device that can shock the heart back to normal beating when athletes suffer cardiac arrest.

He recommended that all large athletic programs take these precautions and said that they should also be used whenever athletes with known heart problems take part in competition.