The 74-year-old checked himself out Friday to make the drive home to Redding, Calif., on his tour bus, but returned a few hours later. Spokeswoman Tressa Redburn says Haggard quickly realized he wasn't up to the trip after checking himself out of the hospital too early.
Haggard is now undergoing a more aggressive antibiotic treatment for pneumonia.
The "Okie from Muskogee" singer had part of a lung removed a few years ago while fighting cancer. He checked himself into the hospital last Tuesday after postponing a show in Macon. He has since postponed the rest of his January shows.
He released his latest album, "Working in Tennessee," last year.