Per the AP's reporting that courtgolf copied in the last post on Seve, I thought that I would do a little research into what exactly is oligoastrocytoma - the subset of tumor that Seve Ballesteros has.

For a more specific definition, I looked over at cancer.gov:

A brain tumor that forms from both oligodendrocytes and astrocytes, which are types of glial cells (cells that cover and protect nerve cells in the brain and spinal cord and help them work the way they should). An oligoastrocytoma is a type of mixed glioma.

Here is the prognosis from Wikipedia.  (Normally, it is a suspect resource, but on medical matters, I would tend to trust it.)

Even after surgery, an oligoastrocytoma will often recur. The treatment for a recurring brain tumor may include surgical resection, chemo and radiation therapy. Survival time of this brain tumor varies - younger age and low-grade initial diagnosis are factors in improved survival time.

A more precise prognosis is from the USUHS (Uniformed Services University of the Health Sciences):

Prognosis is worse with increased age of onset. Oligodendrogliomas have a 5 year survival of greater than 50% and a 10 year survival of 25-34%. The prognosis for Oligoastrocytomas is similar to that for Anaplastic Astrocytomas, which typically have a 3-5 year life expectancy.

This was confirmed by another journal posting.

In effect, it means that Seve is not in one of the demographics of patients that are most likely to survive 1, 5, and 10 years into the future.  But, that doesn't tell you anything about his individual case.