Doctors have put aside ethical concerns to carry out a kidney transplant between two men who met online.
Some doctors say websites subvert equal organ allocation
A hospital in Denver originally postponed the operation amid worries that the recipient might have paid the donor for his organ.
It has now ruled the transplant should go ahead on Wednesday, after both men swore the kidney was not being bought.
The hospital said it was making a "compassionate exception", but that the issue needed urgent debate.
Bob Hickey, who is in need of a new kidney, found a donor, Rob Smitty, using the website MatchingDonors.
The website charges patients up to $290 a month to post their profiles on the internet.
Jeremiah Lowney, medical director of MatchingDonors.com, said the operation was believed to be the first transplant ever arranged through such a site.
But the procedure was called off moments before it was due to take place on Monday when the ethics committee at the Presbyterian/St Luke's Medical Center in Denver had second thoughts.
Mr Smitty said he was glad that the hospital had now changed its mind.
"They're allowing me to do something just good for this
man," he said.
Mr Hickey told Reuters: "This has been an emotional roller coaster."
He said he has had to spend four hours a day on dialysis after losing one kidney to cancer and having the other deteriorate.
Mr Hickey said he had paid Mr Smitty about $5,000, but that this was for his family's trip to Denver and other expenses, as allowed by US law.
The CEO of the hospital, Mimi Roberson, said its decision to go ahead did not mean it was endorsing the website.
There is significant opposition to such resources among the US medical community.
The United Network for Organ Sharing, which matches donor organs to patients, says it subverts the equal allocation of
Mark Yarborough, the director of the Center for Bioethics and Humanities at the University of Colorado Health Sciences Center in Denver, said: "This kind of system potentially may make the overriding criteria the ability to pay."