Willie Nelson cancelled an appearance at a Denver-area benefit concert Saturday and was "suffering breathing problems due to the high altitude and emphysema," according to an announcement made at the show.

An organizer of the concert was later quoted saying Nelson had been hospitalized.

The 79-year-old singer was due to perform at Lulu's Barkin BBQ, a benefit put on at Castles Pines, 20 miles outside Denver, by the Denver Dumb Friends League, a Humane Society Animal Shelter. Although his band arrived for the show, he never did.

Sources: DenverPost and 9News