A brain aneurysm (AN-yoo-riz-um) is a bulge or ballooning in a blood vessel in the brain. It often looks like a berry hanging on a stem. A brain aneurysm can leak or rupture, causing bleeding into the brain (hemorrhagic stroke). Most often a ruptured brain aneurysm occurs in the space between the brain and the thin tissues covering the brain. This type of hemorrhagic stroke is called a subarachnoid hemorrhage. A ruptured aneurysm quickly becomes life-threatening and requires prompt medical treatment. Most brain aneurysms, however, don't rupture, create health problems or cause symptoms. Such aneurysms are often detected during tests for other conditions. Treatment for an unruptured brain aneurysm may be appropriate in some cases and may prevent a rupture in the future. Talk with your caregiver to ensure you understand the best options for your specific needs.