Swiss police say Loretan was leading a client up the summit ridge of the Gruenhorn, in the Bernese Alps, on Thursday afternoon when the pair fell.
The climber died at the scene, while his Swiss client was flown to hospital in a serious condition.
Loretan was one of the few people to have reached the summits of all 14 mountain peaks above 8,000m (26,247ft).
But his career was blighted by a family tragedy just before Christmas 2001, when he killed his own infant son by shaking him to death to stop him crying.
The two climbers fell for unknown reasons at a height of 3,800m, up the 4,043-m peak, police from the Swiss canton of Valais said.
It was a fall of 200m, according to Swiss newspaper La Tribune de Geneve.Manslaughter conviction
Loretan, originally from the canton of Fribourg, began climbing at the age of 11.
He scaled his first 8,000-m peak, Nanga Parbat in Pakistan, in 1982.
Over the next 13 years, he climbed the other 13.
His 1986 ascent of Mount Everest, without bottled oxygen, by night and in just 40 hours, stunned the mountaineering world.
That same year, he climbed 38 Swiss peaks in 19 days, La Tribune de Geneve reports.
He told L'Objectif magazine in 1995 that he had an "extreme fear of death" but his fear kept him crucially alert when climbing.
For the manslaughter of his son of seven months, Loretan was convicted and given a suspended sentence of four months in 2003.
He said he had shaken the child for just a couple of seconds to stop him crying, then put him to bed and the crying had stopped. He later called an ambulance.
The case's notoriety led to new research showing many parents were unaware that infants can die from being shaken for only a few seconds, because of weak neck muscles, the Associated Press news agency recalls.
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