The operation was carried out by surgeons in Kazakhstan
A seven-year-old boy who was admitted to hospital with stomach pains was actually "pregnant" with his twin brother.
Doctors at Chimkent Children's Hospital in Kazakhstan originally believed Mourat Zhanaidarov was suffering from a cyst.
But during surgery, they discovered he was in fact carrying the dead foetus of his twin brother.
The foetus had developed into a tumour but was found to have hair, nails and bones.
'No longer alive'
Doctors at the hospital told the BBC the tumour was the remains of his twin brother's foetus.
They said that while it was no longer a living substance it was feeding off the boy's blood supply.
If left unchecked it could one day have threatened Mourat's own life.
It was remarkable
Valentina Vostrikova, Chimkent Children's Hospital
The tumour has now been successfully removed.
It is believed that the foetus grew inside Mourat while he was developing in the womb.
Doctors believe the two foetuses should have developed into conjoined twins.
"At first we saw that the boy's organs were squeezed by a large hard swelling covered with black hair," one surgeon told a British newspaper.
"As we washed him, we could see that inside him he had his own twin brother. Mourat had been a Siamese twin but nobody knew it.
"Something had gone wrong during his mother's pregnancy and the baby grew inside his brother."
Valentina Vostrikova, chief surgeon at Chimkent Children's Hospital, told the newspaper: "It was remarkable. For seven years it lived like a parasite inside the boy's body."
Doctors at the hospital told the BBC they were unable to explain the phenomenon.
They suggested a number of factors could have played a role, including malnutrition in the mother.