A surgeon at a hospital in Worcester, Massachusetts, is alleged to have performed an operation on the wrong patient, removing their kidney.
The incident is understood to have taken place within the past few weeks at Worcester's St. Vincent Hospital.
Erica Noonan, a spokeswoman for Tenet Health, which owns the hospital, said: "This is a deeply unfortunate situation involving a patient misidentification that took place outside of our hospital and did not involve our employees.
"Our staff followed proper protocols in preparing for and performing the surgery, which was scheduled by the patient's physician at our hospital.
"Saint Vincent Hospital is committed to providing safe, high-quality care to every patient who enters our doors. We are saddened that this incident occurred and our leadership continues to assure the individual receives the support and care needed."
Scott Zoback of the Massachusetts Department of Health told the Worcester Telegram: "We are aware of this serious allegation, and are investigating in line with our state and federal authority."
The US Agency for Healthcare Research and Quality Surgery describes surgery accidentally being performed on the wrong patient a "never event", one of a number of medical errors.
'Wrong-site surgery', a surgical intervention performed on either the wrong patient or the wrong part of a patient's body, is relatively rare. According to a 2006 study, a typical American hospital could expect to experience a case of wrong-site surgery once every five to 10 years.
Between 1985 and 2004, the study identified a rate of 1 in 112,994 cases of wrong-site surgery.