A man suffered first degree burns and heatstroke after a third busy day for medical staff at the Royal Welsh Show.
He was given morphine and intravenous fluids at the event before being transferred to Swansea's Morriston Hospital on Wednesday night.
St John Wales treated 206 people during the day for a variety of injuries, including heat-related problems.
It declared a serious incident and casualties were placed in an inflatable tent to relieve a medical centre.
Meanwhile, a woman was in collision with a bus near the showground in Llanelwedd on Thursday morning at about 10:40 BST. Paramedics were called, but her condition is unknown.
St John Wales said the badly injured man received 40% to 50% burns to his body after being in the sun all day.
Five people were also taken to hospital on Wednesday, three by air.
A woman, who suffered a severe asthma attack, was flown to hospital at Hereford, while a man with cardiac problems was airlifted to Swansea. The third casualty, a man, was treated for severed tendons after a piece of metal pierced his hand.
St John Wales declared a major heatstroke incident after being overwhelmed by people needing help on Monday and Tuesday.
Such was the demand on Tuesday for space that some patients were treated in the centre's kitchen.
An inflatable tent was put up on Wednesday providing an extra 12 spaces.
On Monday staff dealt with 190 cases at the show, while on Tuesday there were 202, of which about 120 were for heatstroke.
Heatstroke cases subsided on Wednesday, with St John staff saying at 15:00 BST that by the end of the day they were expecting to have treated about 100 people, but cases increased later in the afternoon and into the evening.
David Gardner, St John commissioner for mid Wales, said: "Again, a great team effort by all ensured that those who required our assistance were treated appropriately."
The charity is reminding show visitors to wear sensible clothing, to drink plenty of water and use sun cream. It is also providing free sun screen at its show stand.
Medical staff have seen more than 600 people so far this week, compared to 480 last year.
All the St John and Red Cross volunteers have taken annual leave to provide medical assistance at the showground.