An American climber and cancer survivor who had completed the prestigious Explorers Grand Slam has died on Mount Makalu while acclimatizing ahead of a planned summit attempt, Nepali officials confirmed on Monday.

Alexander Pancoe, 39, of Illinois, passed away Sunday night in his tent at Camp II, located at approximately 6,600 meters (21,650 feet) on Makalu. The mountain, which rises to 8,485 meters (27,838 feet), is the fifth-highest peak in the world.

Himal Gautam, director at the Department of Tourism, said Pancoe’s body will be transported to Kathmandu for autopsy before being released to his family.

“He was in his tent after having his dinner. There were no signs of a health problem,” said Iswari Paudel, managing director of Himalayan Guides, the expedition company overseeing his climb. “It was likely a heart attack.”

Pancoe was in the acclimatization phase of his expedition—a critical process climbers undertake to adjust to high altitude before pushing for the summit. He was not using supplemental oxygen.

A brain tumor survivor, Pancoe was diagnosed with a benign tumor at age 18 and underwent surgery at Lurie Children’s Hospital in Chicago. His experience led him to become a passionate advocate for pediatric brain tumor research, raising nearly $500,000 through his mountaineering efforts.

In 2019, he became the 15th American to complete the Explorers Grand Slam—an elite achievement requiring climbers to summit the highest peak on each continent and reach both the North and South Poles. He dedicated his climbs to children undergoing medical treatment, using his platform and initiative “Peaks of Mind” to raise awareness and support.

In 2024, Pancoe disclosed that he had been diagnosed with chronic myeloid leukemia (CML), a type of blood cancer. Undeterred, he continued his advocacy and climbing, attempting Makalu to raise $27,838—matching the mountain’s height in feet—for pediatric blood cancer programs.

The U.S. Embassy in Kathmandu and Pancoe’s family have been informed of his passing.

Nepal, home to eight of the world’s 14 peaks above 8,000 meters, is currently in the midst of its spring climbing season. More than 1,000 climbers have received permits to attempt various Himalayan summits, according to the Department of Tourism.

Pancoe’s death is among the first confirmed fatalities of the spring climbing season. Earlier, two Sherpa climbers went missing on Annapurna after being swept away by an avalanche near Camp III. The pair, among the most skilled and experienced mountaineers in Nepal, have not been found and are presumed dead.