Narrator: This is Science Today. For years, doctors have debated the value of surgically treating all unruptured cerebral aneurysms. While most do require surgical treatment, a new University of California, San Francisco study suggests that in a large subset of patients, such treatment is unnecessary. Dr. Clay Johnston, an assistant professor of neurology was lead author of the study.

Johnston: Previously, people, the numbers they had found were around one - one of those hundred would have a rupture in one year. And clearly at those levels, treatment was justified. But the new study's much larger than the previous ones, had found that in sub groups of patients, in ones with very small aneurysms where they hadn't had a ruptured aneurysm elsewhere, that their rupture rates were very, very low - in fact, one twentieth of one percent a year.

Narrator: This new information is reassuring to patients who had formerly thought of unruptured aneurysms as time bombs.

Johnston: They don't have a time bomb and they can go on and live their life normally and not necessarily expect to have any problems whatsoever from the aneurysm.

Narrator: For Science Today, I'm Larissa Branin.