Saudi King Abdullah, hospitalised earlier this week, is suffering from pneumonia and breathing with the aid of a tube, but is in stable condition, the royal court said on Friday.
The long-ailing king, who is believed to be around 90 years old, was admitted to the King Abdulaziz Medical City in Riyadh on Wednesday for checks.
Examination "revealed pneumonia, which required the provisional insertion of a tube on Friday evening," a statement said.
"Thanks be to God, that step was crowned with stability and success," the statement added, without saying how long the king would need to remain in hospital.
In recent years, his advanced age and poor health have raised concerns about the future leadership of one of the world's key oil producers.
Abdullah's half-brother Salman, 77, is next in line to the throne. He was named crown prince in June 2012 following the death of Prince Nayef bin Abdul Aziz.
Salman has been representing the king at most recent public events, including the latest Gulf summit in Qatar last month, because of the monarch's ailing health.