The death in New York of Saudi Arabia's King Abdullah younger half-brother Crown Prince Sultan, announced early on Saturday, means King Abdullah will now have to nominate a new heir, widely expected to be the interior minister, Prince Nayef.
King Abdullah had a back operation on Monday, almost a year after undergoing two rounds of surgery to treat a herniated disc that kept him out of the kingdom for three months.
The softly spoken Abdullah was born in the court of his father, King Abdulaziz Ibn Saud, in the early 1920s, when the capital Riyadh was a small oasis town ringed by mud-brick walls at the centre of an impoverished but rapidly growing kingdom.
After becoming de facto regent as crown prince when King Fahd had a stroke in 1995, he enacted reforms aimed at reconciling Saudi Arabia's conservative traditions with the needs of a modern economy, a process he extended on becoming king in 2005.
As head of OPEC's biggest producer, he pursued a moderate oil price policy, raising production to prevent price spikes during supply outages in other countries.
But King Abdullah opposed the pro-democracy demonstrations of the Arab Spring, reflecting Saudi concerns that the fall of old allies might give openings to regional rival Iran and to al Qaeda.
Before becoming king, Abdullah opened up the economy to private and foreign investors, reduced clerical control over girls' education and pursued changes to the Islamic judiciary.
The reforms were spurred by a need to address unemployment by strengthening the private sector and better preparing young Saudis for jobs, and to reduce the influence of Islamist militants who had supported al Qaeda's three-year bombing campaign in the kingdom.
But the reforms almost entirely avoided the issue of political change, and the only elections in the kingdom are for half the seats on town councils that have little power.
Some activists who have demanded change in petitions ended up in prison, and political parties and public demonstrations are banned.
King Abdullah has also aimed to improve the position of women in his ultra-conservative country, trying to offer them better education and employment prospects and saying they will be allowed to take part in municipal elections in 2015.
Women are still barred from driving and must seek the approval of a male "guardian" to work, travel abroad or undergo surgery in some cases.
When the Arab Spring rippled across the region early this year, the king's order to spend $130 billion on social benefits, new housing and new jobs helped avert any significant pro-democracy unrest in Saudi Arabia.
In a ruling family known for lavish excesses, King Abdullah's fondness for retreats at his desert camp has distinguished him from Saudi princes who prefer to spend summers in Mediterranean palaces.
In recent years, the king's foreign policy has increasingly focused on efforts to contain what the Sunni monarchy sees as the increasing influence of Shi'ite Muslim power Iran across the Arab world.
That policy reached its high point in March when Saudi Arabia sent troops to Bahrain to support the island's Sunni Muslim monarchy against an uprising by the Shi'ite majority.
Riyadh feared that the U.S. invasion of Iraq in 2003 had already altered the regional balance of power, giving Iran more sway from Beirut to Baghdad.
Those concerns were underpinned by Iran's development of a nuclear power station, which the West suspected of hiding an atomic weapons programme.
In a 2009 diplomatic cable released by WikiLeaks, King Abdullah was quoted repeatedly as urging the United States to "cut off the head of the snake" by attacking Iran.