Euthanasia, which was legalized in Belgium a decade ago, has for the first time allowed a gravely ill prisoner to die under the law, according to reports.
The man, who was not identified, died earlier this year after seeking permission several times when doctors decided his suffering could not be helped, the reports said.
They gave no details of his illness or the circumstances of his case.
Another prisoner, locked up for 27 years for committing two murders, has also asked for permission to die and should satisfy the legal conditions, Flemish television channel VTM said.
After the Netherlands, Belgium was the second country in the world to legalize euthanasia in 2002.
Last year, some 1,133 cases -- mostly for terminal cancer -- were recorded, about one percent of all deaths in the country, according to official figures.
The issue is very controversial and most countries forbid mercy killing but some allow "passive" euthanasia when a doctor is not required to fight to save a life and there are others where patients can refuse medical care.