A woman donated a kidney to a complete stranger, undergoing surgery and a battery of physical and psychological tests simply because she knew there was a long waiting list for organs.
Experts said that transplants in which neither donor nor recipient knows the other person were almost unheard of until a few years ago.
In this case, the 50-year-old donor was "purely motivated by altruism," said Cheryl Jacobs, a social worker who took part in the psychological screening of the woman.
The donor and the recipient both asked to keep their identities secret, even from each other.
The transplant took place in early August but wasn't disclosed until Thursday after the patients had left the hospital at the University of Minnesota.
Transplanted kidneys generally come from a cadaver or a living relative. Transplant programs have been reluctant to accept kidney donations from living strangers for several reasons, including concern about exposing the donor to the risks of surgery.