A "very grateful" former patient has sent NHS staff at a hospital in Lincolnshire an anonymous Christmas card every year since 1960.
Stamford Hospital said the mysterious unsigned cards appear on the Matron's desk "every year without fail".
One of the cards reads: "From a very grateful patient in 1960! And still going strong."
Staff are now hoping to track down the sender so they can thank them for their festive messages.
Sue Brooks, Matron at Stamford Hospital, said: "It would be great to find who is still so grateful after all these years.
"His gratitude has spanned over 57 years and it would be so lovely to add to the already growing history about this hospital.
"This has been a great mystery for staff at Stamford Hospital since 1960 who are eager to find out the mystery sender of the Christmas card so they too can say t'hank you and Merry Christmas'."
The hospital says the cards are addressed to the now disused Exeter ward, which was once a busy male-only ward, suggesting the sender is a man.
It says the postal mark of the cards is for the South East Anglia Mail Centre, which is based in Chelmsford, Essex.
"The Christmas card appears every year without fail on the Matron's desk at Stamford Hospital and the patient must have had good cause to be grateful for the treatment he received," a spokesperson for the North West Anglia NHS Foundation Trust, which runs Stamford Hospital, said.
"Members of staff who worked at the hospital in 1960 and the current staff are all intrigued about the card and why he feels the need to say thank you after all these years. Can you help to solve the mystery for our staff?"