Sarah Murnaghan, who turned 11 on Aug. 7, suffers from end-stage cystic fibrosis. She spent six months in the hospital after receiving a life-saving double lung transplant.
The little girl was originally denied access to be placed on the list for adult lung transplants because of her age.
She received two lung transplants this summer, after a federal judge intervened in her parents' lawsuit challenging national transplant rules. Sarah's first set of adult lungs failed after a June 12 transplant. A second set was transplanted three days later.
Sarah's release caps days of upbeat progress reports from her family.
On Sunday, her mother Janet Murnaghan said her daughter was taken off oxygen, although she still gets support from a machine that helps her breathe. She said Sarah also started walking with the aid of a walker, even venturing outside.
A family spokesman said Sarah's recovery will now be focused on building her muscle strength so she no longer has to use a breathing tube. The spokesman said Sarah recovered from a case of pneumonia that stemmed from the tube.
Sarah's case has raised questions among some health specialists and medical ethicists about how organ donation rules are developed and under what circumstances they might be disregarded.
Anne Paschke, a spokeswoman for the United Network for Organ Sharing (UNOS). said that the temporary option for other children to apply for adult lungs will be in effect until the end of June 2014. She said a committee is currently studying whether to make that change permanent, or make other changes to adolescent lung transplant rules.
So far, six children, including Sarah, have applied for adult lung transplants as a result of the temporary rule, Paschke said. Two of the others ultimately received adolescent donor lungs and three are still waiting for transplants.
The Associated Press contributed to this report.