At the 2004 US Olympic Trials, Hoelzer qualified to swim the 200 m backstroke by placing first, with a time of 2:11.88. She also swam in the 100 m backstroke, but did not qualify to swim that event at the Olympics.
At the US Trials on July 1, 2008, Hoelzer qualified to swim in the 100 m backstroke at the 2008 Olympics. On July 5, 2008, Hoelzer broke her former Auburn University roommate Kirsty Coventry's world record of 2:06.39 in the 200 m backstroke with a time of 2:06.09, ultimately qualifying for her second event in the 2008 Olympics. This record was broken in Beijing by Coventry. She also qualified for the women's 4x100 medley relay.
Hoelzer won a bronze medal in the 100 m backstroke on August 12 in Beijing. She also won the silver medal in the 200 m backstroke on August 16, and the silver medal in the 4x100 m medley relay on August 17 after swimming in the qualifying heats for that event.