A man who died in Swansea had been suffering from measles at the time, says Public Health Wales.
The 25-year-old, named locally as Gareth Williams, was found dead at a flat on Thursday.
Public Health Wales (PHW), which is tackling an epidemic of more than 800 cases, said tests confirmed only that the man had measles at the time of his death.
A post mortem examination has yet to take place to confirm cause of death.
"Public Health Wales laboratory tests have today confirmed a diagnosis of measles in a 25-year-old male from Swansea who died on Thursday 18 April," said Dr Marion Lyons, PHW's director of health protection.
Further investigations are being undertaken by the Swansea coroner to establish the cause of death.
If confirmed, it would be the first death in the measles epidemic.
"My sympathies are with the family at such a tragic time," said Dr Lyons.
"Whatever the cause of death in this case we should not be surprised if, as the outbreak grows, we start to see deaths in Wales".
Mr Williams' body was discovered in a flat at Port Tennant Road, Swansea at about 08:15 BST on Thursday.
Earlier, police called the death "sudden and unexplained".
There have been 808 cases of measles confirmed so far in the Swansea epidemic, which also includes the neighbouring Neath Port Talbot and Bridgend areas, and further into mid and west Wales.
MMR jabs are being offered in schools although initial take-up was said to be "disappointing" by Abertawe Bro Morgannwg health board.
There will also be drop-in sessions for the third Saturday in a row at Morriston and Singleton Hospitals in Swansea, the Princess of Wales Hospital in Bridgend and Neath Port Talbot Hospital.
Last weekend around 2,500 people received the MMR vaccination at special clinics held across south Wales.
Dr Brendan Mason, a consultant epidemiologist with PHW, said he could not comment on Gareth Williams' death as it was still being investigated.
He told BBC Radio Wales that there was a "real possibility" that every measles case could end up in hospital, in intensive care, or have a worse outcome.
"Anyone born after 1970 should make sure they are immunised, especially those in the 10-18 year-old age group," he said.
"Lots of people are not taking it seriously and we have to continue to promote immunisation, not just to protect individuals but also their families and the wider community."
Dr Lyons added: "Measles is a potentially fatal disease and around one in every 1,000 people who contracts measles in developed countries will die.
"Those not fully vaccinated with two doses of MMR are highly likely to catch measles, which is highly contagious."
Symptoms of measles include fever, cold-like symptoms, fatigue, conjunctivitis and a distinctive red-brown rash that appears a few days into the illness.
A Welsh government spokesman added that it was aware of a possible death from measles and extended "deepest sympathies to the family and friends at this difficult time".
He added: "We continue to be in close contact with Public Health Wales and to stress the importance of vaccination as the most effective way to protect against measles."
Although the outbreak was at present affecting the Abertawe Bro Morgannwg, Powys and Hywel Dda health board areas, there have been cases of measles in every health board area in Wales.
On Friday, Betsi Cadwaladr University Health Board in north Wales urged parents to get their children vaccinated, reminding people of an outbreak in Porthmadog which resulted in more than 60 people contracting measles in 2012.