Saudi Arabia's King Abdullah arrived at hospital in Riyadh on Sunday to undergo back surgery, the world's top oil exporter said.
The king arrived at the King Abdulaziz Medical City in Riyadh earlier on Sunday, the state news agency SPA said.
The ruler of the conservative Islamic kingdom, who is thought to be 88, suffered a herniated disc late last year requiring surgery in the United States.
On Tuesday the conservative Islamic kingdom said Abdullah would undergo surgery in the coming days to tighten the connectors around his third vertebra.
King Abdullah's health has been a matter of keen interest in the conservative Islamic kingdom, where the system of politics is strictly hierarchical and doubts linger over the ruling family's long term succession plans.
His heir is Crown Prince Sultan, who is in his mid 80s, has been in the United States since June seeking medical treatment, and was described in a 2009 U.S. diplomatic cable released by WikiLeaks as "to all intents and purposes incapacitated".
The next-in-line after Sultan is widely assumed to be Prince Nayef, the interior minister since 1975, who is in his late 70s and has a reputation as more conservative than his elder brothers the king and crown prince.
Unlike in European monarchies, the line of succession does not move directly from father to eldest son, but has moved down a line of brothers born to the kingdom's founder Ibn Saud, who died in 1953.
So far five brothers have become king and around 20 are still alive, but only a few of those are thought realistic candidates to rule Saudi Arabia.
Abdullah became king in 2005, but had already ruled as de facto regent since King Fahd suffered a stroke in 1995.
The leader of the West's foremost Gulf ally has given Saudi backing to US-led efforts to confront and constrain Islamist militant groups including Al Qaeda and has pushed Washington to support greater rights for Palestinians.
Another US diplomatic cable released by WikiLeaks said the king had pushed the United States to take a harder line against Iran, which the West suspects of planning to build an atomic bomb. Tehran says its nuclear programme is purely for peaceful purposes.
This year he disagreed with Washington over the Arab Spring protests in other Arab countries by supporting the ousted leaders of Egypt and Tunisia and sending troops to Bahrain to help its ruling family quash a Shi'ite uprising.
He has pushed cautious domestic reforms aimed at liberalising Saudi Arabia's economy, giving greater technical, rather than religious, emphasis to education, and allowing women more rights.
Last month he said women would have the right to participate in future municipal elections for the first time, but they can still not legally drive and need the permission of a male relative to travel outside the kingdom, work or have some kinds of surgery.
King Abdullah is seen to have supported a moderate oil policy, raising Saudi crude production to prevent price spikes during supply outages from other countries.