Canadian Scott Routley, from London, Ontario, communicated with researchers via a brain scan, proving that he is conscious and aware.
It is the first time such a severely brain damaged patient has been able to provide clinically relevant information to doctors.
Neuroscientist Professor Adrian Owen, who leads the research team at the Brain and Mind Institute of Western Ontario, said: "Scott has been able to show he has a conscious, thinking mind. We have scanned him several times and his pattern of brain activity shows he is clearly choosing to answer our questions."
Prof Owen was speaking on a BBC Panorama programme to be broadcast tonight.
He said the breakthrough could lead to improvements in the treatment of severely brain damaged patients who cannot move or speak.
"Asking a patient something important to them has been our aim for many years," he told the programme.
"In future we could ask what we could do to improve their quality of life."
Prof Owen's team uses functional magnetic resonance imaging (fMRI) brain scans to detect hidden awareness in patients and open up channels of communication.
The scans produce images of "active" regions of the brain by tracking the flow of oxygen-rich blood. Patients are asked to imagine playing tennis or walking around their home -- two thought processes that produce distinct patterns of activity in different parts of the brain.
By monitoring the activity on an fMRI scanner, the researchers can ask yes or no questions. One type of brain activity is taken as a "yes" and the other as a "no".
Mr Routley suffered traumatic brain injuries when his car was in a collision with a police vehicle. Until Prof Owen's intervention, he was assumed to have been in a vegetative state for more than 12 years.
Neurologist Professor Bryan Young, from University Hospital in London, Ontario, has cared for Mr Routley for 10 years. He said the scan results overturned all previous assessments of the man's condition.
"He had the clinical picture of a typical vegetative patient -- no emotional response, no fixation or following with his eyes.
"I was quite amazed that he was able to show these cognitive responses with fMRI."
Prof Owen has previously shown that nearly one in five vegetative patients may in fact be conscious.