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Willie Nelson was recently admitted to a Denver hospital after suffering respiratory issues, ET has learned.

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The country music star, 79, was scheduled to perform at a fundraiser in Castle Pines Village for the Denver Dumb Friends League on Sunday afternoon, but the singer failed to show. Concert-goers were then notified that Nelson was experiencing "breathing problems due to the high altitude and emphysema."

A host of the event, Fred Bartlit, told The Denver Post he was advised that Willie "woke up and had trouble breathing," therefore would not be attending the scheduled concert.