Are brain aneurysms usually fatal?

Sometimes: There is good amount of study performed on the natural history of brain aneurysms. Aneurysms under 5 mm tend to have a low rate of rupture an hence not very dangerous. Conversely, the larger aneurysms (specially those larger than 1 cm) are very dangerous and every time they bleed kill 50% of the patients. Hence, if your aneurysm has not ruptured, it is important to discuss the risks with a md.

Ruptured or not?: Ruptured brain aneurysms can cause devastating neurological injury with up to a 50% mortality (death) rate if left untreated. Non-ruptured aneurysms generally have little or no symptoms and may be treated electively if they are large enough (>5mm), are growing, or have a very irregular shape. The precise size of non-ruptured aneurysms which require treatment is controversial.

Sometimes: If a brain aneurysm ruptures, there is a 40% of death before you make it to the hospital and a 50% chance of poor outcome or death of those who do make it to the hospital.