Saudi Arabia's Crown Prince Sultan bin Abdul Aziz has returned home from Morocco, as King Abdullah is due to fly to the US for back treatment.
The king, aged 86, has a herniated disc worsened by an accumulation of blood around the spine.
The crown prince, who is in his early 80s and has been treated for what is said to be cancer, is expected to take up the reins during the king's absence.
Saudi Arabia has more than 25% of the world's known oil reserves.
The desert kingdom is also a key ally of the US in the Middle East.
For years, power in the country has been passed between the many sons of the country's founder, King Abdul-Aziz, who died in 1953.
The kingdom has no elected parliament or political parties.
The issue of handing power to a younger generation has become more pressing as health issues affect the brothers and half-brothers, says the BBC's Middle East correspondent Jon Leyne.
Last week, King Abdullah handed over control of the country's National Guard to his son Mitab.
King Abdullah came to power in 2005 after the death of his half-brother King Fahd. He had already been de facto ruler since King Fahd had a stroke in the 1990s.