Subarachnoid hemorrhage: Bleeding within the head into the space between two membranes that surround the brain. The bleeding is beneath the arachnoid membrane and just above the pia mater. (The arachnoid is the middle of three membranes around the brain while the pia mater is the innermost one.) The subarachnoid space is a potential space. It is normally filled with cerebrospinal fluid. With a subarachnoid bleed, the cerebrospinal fluid in the subarachnoid space is bloody. Subarachnoid hemorrhages are typically acute (sudden). They may follow a head injury or rupture of a blood vessel in the central nervous system (brain and spinal cord) as, for example, because of an aneurysm in the vessel. Nearly half of people admitted to a hospital with a subarachnoid hemorrhage die within a month. Many of the survivors are left with severe disabilities. The first-degree relatives of patients with subarachnoid hemorrhage are more likely to develop the condition than those who do not have a family history of it. Smoking, alcohol abuse, and high blood pressure are risk factors for subarachnoid hemorrhage.