Nagoya University Hospital said Wednesday one of its surgical teams left gauze inside a woman's body during an operation carried out 44 years ago.
The patient, an Aichi Prefecture resident now in her 80s, complained of stomach pain for many years.
The thin fabric was found in 2014 when the hospital performed an operation to remove a tumor from the woman's rectum. The woman had been diagnosed with a tumor in her pelvis at the hospital in April that year.
After doctors cut into part of her rectum during surgery, a cloth-like object was found with the tumor, according to the hospital.
The hospital admitted the possibility that the stomach pain or tumor might have been caused by the gauze.
The woman underwent the 1970 surgery to treat infertility. As she never had another operation until the recent surgery, the hospital concluded there is a high probability that the doctors forgot to remove the gauze.
The hospital has apologized to the woman and completed paperwork to compensate her. It revealed the malpractice after gaining the woman's consent.