57-year-old David Bennett, who doctors say was too ill to qualify for a human heart, is doing well three days after the experimental seven-hour treatment.
The surgery is being hailed by many as a medical breakthrough that could shorten transplant waiting times and change the lives of patients around the world. But some are questioning if the procedure can be ethically justified.
They have pointed to potential moral trouble spots over patient safety, animal rights and religious concerns.
So how controversial are transplants from pigs?