In 1966, Mr Walters, of Crowthorne, Berkshire was left in agony when his back gave way as he loaded heavy luggage into the back of his car.
He faced a trip to the doctors and possible confinement to a wheelchair.
But as he laid in bed that night, someone entered his room and told him to follow them.
When he awoke his back had been cured, so he went to Heatherwood Hospital to ask for the surgeon who operated on him.
But staff denied any knowledge of it. Further tests showed he had undergone then-impossible back surgery.
Not only that, but his blood makeup matched an astronaut who had spent 10 hours in space. Doctors said the only rational explanations for this could be if he were a lifelong smoker or had visited polluted Mexico City - but he had done neither.
A Harley Street specialist has since told him of other people who had experienced interstellar medical procedures.
He also said Mr Walters would later probably find other operations he did not remember. Mr Walters revealed he later discovered a further back operation and ear surgery. He added his claims have been supported by University of Connecticut scientists.
Mr Walters said: "Before the operation, if someone had spoken about UFOs I would have thought they were mad.
"But it is the way we are brought up, we are taught there must be an explanation for everything.
"Ninety per cent of me still doesn't believe what happened to me, but there is 10% that neither I, nor anyone in the medical profession, can explain.
"I am not airy-fairy about this. I don't sit in my garden at night waiting for another UFO to appear, but it has certainly made me believe that there is extra-terrestrial life."