Seve Ballesteros is scheduled to undergo another operation on a brain tumour.
The operation tomorrow will attempt to relieve pressure on his brain caused by an edema and bleeding that have recently developed.
The remaining parts of the tumor, located in a very deep part of the brain, will be removed in the operation, a statement said.
A study of cells already removed has revealed the tumor is classified as an oligoastrocitoma, a type that affects "cells that cover and protect the nerve cells in the brain and spinal cord," the statement said.
The La Paz hospital said the operation is "of great complexity."
Ballesteros was admitted to the hospital on October 6, and 10 days later a sizable part of the tumour 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 Opens and two Masters, briefly lost consciousness while at Madrid's international airport on Oct. 6 and was admitted to the hospital.
In a personally signed statement from his hospital bed, Ballesteros said he faced the "hardest challenge of my life."