Friends said Monday's surgery at Bakersfield Memorial Hospital was a success and that the 71-year-old country crooner was in stable condition.
Doctors informed Haggard, who's from a Bakersfield suburb, about a spot on his lung in May. He told The Bakersfield Californian in August that he wasn't going to seek treatment, but friends and family members convinced him otherwise.
Haggard became a household name after the anthem "Okie From Muskogee" hit No. 1 in 1969. He has had 38 No. 1 hits stretching back to 1966.