Seve Ballesteros' brain tumor is cancerous and he will undergo another operation.
La Paz Hospital said Thursday that the surgery is "of great complexity." The procedure Friday is designed to relieve pressure on his brain caused by swelling and bleeding that have recently developed.
The remaining parts of the malignant tumor, located in a very deep part of the brain, will be removed.
The 51-year-old Ballesteros was in stable, but serious, condition before the operation, the hospital said.
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Based on an analysis of cells already moved, the tumor is classified as an oligoastrocytoma, a type that affects "cells that cover and protect the nerve cells in the brain and spinal cord," the hospital said.
Ballesteros was admitted to the hospital Oct. 6, and 10 days later a sizable part of the tumor was taken out.
Part of his skull was removed -- a procedure known as decompressive craniotomy -- to allow room for a swelling brain to expand. Doctors said it was not uncommon after such complex operations.
Ballesteros, who won three British Opens and two Masters, briefly lost consciousness while at Madrid's international airport Oct. 6.
In a personally signed statement from his hospital bed, Ballesteros said he faced the "hardest challenge of my life."
Ballesteros, who won a record 50 tournaments on the European Tour, retired last year because of a long history of back pain and has since concentrated on golf course design.
Ballesteros transformed European golf. After the Ryder Cup was expanded to include continental Europe in 1979, Ballesteros helped beat the United States in 1985 to begin two decades of dominance. He also captained Europe to victory in 1997 at Valderrama, Spain.
Ballesteros and Jose Maria Olazabal made one of the most formidable partnerships in Ryder Cup history, with 11 wins, two losses and two halves.