The numbers are grim. According to the United Network for Organ Sharing, more than three times as many people in the United States were waiting for heart, kidney and liver transplants than received them in 1999; over 5,500 people died waiting for organs. And many more patients could benefit from organ transplants than ever make the waiting list, either because they are too sick or too healthy to qualify. According to one estimate, 45,000 Americans could benefit each year from heart transplants, but only 2,000 or so human hearts are available.

One promising solution to this medical predicament is to harvest organs from suitable animals and use them for human transplant. It may sound outlandish, but several biotech and pharmaceutical heavyweights, as well as some smaller biotech firms, are gearing up to do just that (see table, next page). In preliminary experiments, these companies have already implanted animal cells in human volunteers to treat such diseases as Parkinson's and epilepsy. Researchers plan to start whole-organ clinical trials in the next two to three years.

The use of animal organs in human transplants is not exactly new. You may remember "Baby Fae," the 12-day-old California infant who, in 1984, received a heart transplant from a most unusual donor-a baby baboon. This controversial experiment marked the first time most people had ever heard of "xenotransplantation," transplanting tissues or organs between species, and Baby Fae's very public death only 20 days after her surgery chilled the climate for xenotransplantation work. Still, medical researchers quietly pressed ahead, and their efforts may soon pay off.

The animal of choice in the new generation of experiments? Pigs. That's because pigs are both plentiful and easy to raise, and "the similarities to man are amazing," says Julia Greenstein, CEO of Immerge BioTherapeutics, a joint venture between Swiss drug giant Novartis and Charlestown, MA-based BioTransplant.