Pop star Lady Gaga fell ill in Denver, Colorado on Wednesday night (06Aug14) when she was hit with a bout of altitude sickness.

The Born This Way hitmaker brought her artRAVE: The ARTPOP Ball world tour to the city's Pepsi Centre on Wednesday night but the city's high elevation took its toll on the singer.

After the show, Gaga posted a picture of herself breathing through an oxygen mask on photo-sharing site Instagram.com, writing in the caption, "Altitude sickness is no joke!... #artRaveDenver many true ravers crowd tonight."

Denver is located on the edge of Colorado's High Plains and is nicknamed the Mile-High City because it is exactly a mile above sea-level.