A fertility clinic in Ishikawa Prefecture admitted Saturday it had transplanted a fertilized egg into the wrong patient in 1995.
The clinic said the woman did not become pregnant from the error, which occurred in March 1995.
The Japan Society of Obstetrics and Gynecology said it is the first time that such a mixup has been reported in Japan. In vitro fertilization was first introduced in Japan in 1983.
The clinic said the egg was transplanted into a woman in her late 20s from neighboring Toyama Prefecture, who had a name similar to the woman whose egg she received.
The mixup occurred after the doctor called the donor in from the waiting room to receive the treatment. Mistaking the names, the woman from Toyama entered instead, the clinic said.
The clinic noticed the mistake soon after surgery and offered the woman an apology and promised her an abortion if she became pregnant, and offered free treatment until she became pregnant, it said.
It also took measures to prevent a recurrence, such as confirming full names, addresses and other patient information before surgery, it said.
The treatment used to be available at only a limited number of institutions in Japan. In 1998, however, the therapy was conducted at 442 facilities across the nation, resulting in 10,986 test-tube babies, or one in every 100 newborns, the society said.