While leading Mountain Madness, Fischer became renowned for his ascents of the world's highest mountains without the use of supplemental oxygen. He and Wally Berg were the first Americans to summit Lhotse, the world's fourth highest mountain (27,940 feet / 8516 m), located next to Mount Everest. Fischer and Ed Viesturs were the first Americans to summit K2 (28,251 feet / 8611m), in the Karakoram of Pakistan, without supplemental oxygen.