People can live normal, healthy lives with just one kidney
Twenty-two people have given a kidney to a stranger since the practice became possible in the UK two years ago, the body in charge of such donations says.
Ten put themselves forward in the first year and 15 in the second - three of these have not yet undergone surgery.
The Human Tissue Authority (HTA), which decides whether people are suitable, said the numbers were "remarkable".
There are currently 7,000 people waiting for a kidney in the UK amid a serious shortage of donor organs.
To help tackle this shortage, the HTA changed the rules at the end of 2006 to allow those who were not related - either genetically or through marriage - to become living donors.
"We expected to see a small number of cases when we first started approving this type of transplant, but we did not expect to see the number rise so significantly after just one year," said Vicki Chapman, director of policy and strategy at the HTA.
"Donating a kidney to someone you do not know really is an altruistic act; the medical tests take time and the procedure is not without risk.
The donors I have come across are genuinely altruistic, they decide that, on balance, donating is unlikely to do them any harm but will transform someone else's life
"It is the role of the HTA to ensure that those giving so generously fully understand the risks involved."
The rates of living donations among relatives have also increased, with 1,008 such donations approved in 2008/2009 compared with 961 the previous year.
More than 1,000 other transplants were carried out using organs from deceased donors.
Living donations offer a number of advantages over organs given after death.
The quality of the organ is likely to be higher and surgery can be planned in advance.
Evidence suggests that the long-term success rate for the recipient is higher when he or she has received a living organ.
As with all surgery, the operation for the donor is not risk-free and they must undergo a series of rigorous checks in the run-up to the donation.
Assessments are also carried out to ensure that they are not under any pressure to donate, and consent is given freely and voluntarily.
Then, if they are found to be a suitable donor, they will be matched with a recipient.
There are no long-term consequences to living with just one kidney, the organ responsible for removing waste from the body.
When one of the pair is removed, the remaining organ simply increases in size and capacity to compensate.
Plymouth Hospitals NHS Trust has carried out three of the 22 altruistic transplants.
"The donors I have come across are genuinely altruistic, they decide that, on balance, donating is unlikely to do them any harm but will transform someone else's life," said consultant nephrologist Peter Rowe.
"It would have been difficult for a living person to donate an organ altruistically before the HTA code of practice was published, but now we have a robust regulatory framework to work to."
A spokesman for Kidney Research UK said: "We welcome the figures but obviously there's still more that should be done to encourage organ donation, especially when you consider that 7,000 people are currently in need of a kidney."