BOSTON, DEC. 19 -- Treatment of heart transplant patients with a powerful immune-suppressing drug appears to sharply increase their risk of a lethal form of blood cell cancer, a study has concluded.

Researchers found that 11 percent of patients at one hospital who were treated with OKT3, an antibody that attacks white blood cells that play a key role in making the body reject the transplanted organ, quickly developed lymphoma, and most of them died.

The new study shows that even if OKT3 prevents rejection -- and this is in doubt -- the benefit comes at the high risk of cancer.

"We found that the addition of OKT3 was statistically strongly linked with a sharp increase in this form of cancer," said Lode J. Swinnen of Loyola University Medical Center in Maywood, Ill.

People who get transplanted organs routinely receive a variety of medicines that temporarily weaken their immune defenses. This is intended to thwart the body's natural tendency to attack the alien organ.

Doctors have long recognized that these medicines increase the risk of blood cancer. However, Swinnen's study, published in Thursday's New England Journal of Medicine, found that OKT3 heightened the hazard dramatically.

The cancers in those who received OKT3 at the time of their transplant typically developed a month or two after the surgery and were rapidly fatal.

OKT3 gained widespread use about three years ago. Of 79 heart transplant patients who got the treatment at Loyola, nine developed lymphoma, and six of them died.

Swinnen said that physicians at Loyola have stopped using OKT3 to prevent rejections. But it is still used as a treatment once rejection occurs, since patients will die unless their transplanted hearts can be saved.

He said that although OKT3 is widely used to prevent rejection, there is little good evidence that it works.

In an editorial in the journal, Israel Penn of the University of Cincinnati Medical Center said the use of these drugs and the development of cancer "indicate that we have much to learn about walking the tightrope between preventing rejection and inducing lymphomas."

However, he noted that overall, only about 1 percent of all transplant recipients die of cancer.