Heart bypass surgery is significantly more risky for women than men, says a massive study that found women die from the operation up to twice as often as men who are just as old and sick when they go under the knife.

Now doctors must figure out why. "I don't know the answer, and I don't think anyone does," said Dr. Fred Edwards, a cardiothoracic surgeon at the University of Florida, who reported the bypass findings Wednesday in the Annals of Thoracic Surgery. Now he's preparing to research just what kills these women so that perhaps it can be prevented.The risk of death from bypass surgery, immediately or within a few days, is very low for both men and women, so the study shouldn't scare anyone away from what often is a lifesaving operation, said Dr. Sidney Smith, past president of the American Heart Association. But Smith said the finding that women fare worse is disturbing and must be explored quickly.

"The next step is very much to understand whether the cause of death is different, because that could hold the clue as to why it's occurring," said Smith, cardiology chief at the University of North Carolina, Chapel Hill. "Heart disease in women is an issue we really need to understand a lot more about."