A patient was injured following a fire at the University Hospital of Wales in Cardiff in the early hours of Thursday morning.
Nurses and security staff were alerted to a small fire on ward C5 South.
It is understood that the fire started after a patient lit a cigarette at his hospital bed.
The patient is now believed to be in intensive care.
The health board wasn't able to confirm whether this was the cause of the fire.
The man, who has not been named, is currently being treated for his injuries at the hospital.
Two wards were evacuated but patients were unharmed.
Patients from C5 North have returned to their ward, but patients from C5 South remain in a separate area while the investigation into the fire continues.
C5 is understood to treat patients with lung and heart conditions.
A spokeswoman for Cardiff and Vale University Health Board said: "This was an isolated incident and thanks to the swift actions of our staff was contained to a side room of the ward; the exact cause and nature of the incident is being investigated by the South Wales Fire and Rescue Service.
"We would like to thank our staff who were on duty on the ward this morning, whose swift and brave actions ensured that the incident was contained and the majority of patients on the ward were unharmed."
South Wales Fire and Rescue Service and South Wales Police confirmed they were both in attendance.
