1. The surgical removal of half the brain.
2. Excision of one cerebral hemisphere; undertaken for malignant tumors, intractable epilepsy usually associated with infantile hemiplegia due to birth injury, and other cerebral conditions.

Surgeons have performed hemispherectomies hundreds of times for disorders that can not be controlled any other way

• The surgery has no apparent effect on personality or memory.
• People can survive and function fairly well after the procedure although they may have some physical disabilities; such as, a significant loss of the function on one side of the body.
• A person can walk, run, dance, or skip; but some lose use of the hand opposite the hemisphere that was removed.
• Sometimes, if the left side of the brain is taken out, most people have problems with their speech; however, the younger a person is after a hemispherectomy, the less speech disability the person is likely to have.
• The surgery consists of two forms: "anatomical hemispherectomic removal" of an entire hemisphere of the brain or "functional hemispherectomies" that take out only parts of a hemisphere.
• Doctors often prefer anatomical hemispherectomies because "leaving even a little bit of brain behind can lead seizures to return", stated neurologist John Freeman of Johns Hopkins, which specializes in the procedure.
• The functional hemispherectomies, which U.C.L.A. surgeons usually perform, lead to less blood loss.
• A recent study found that 86 percent of the 111 children who underwent the procedure at Johns Hopkins between 1975 and 2001 are either seizure-free or have non-disabling seizures that do not require medication.
• One reason for the procedure is to stop debilitating seizures and today brain surgeons perform hemispherectomies on patients who undergo dozens of seizures daily which resist all medication and result from conditions that primarily afflict one hemisphere of the brain.
• The seizures are often progressive and damage the rest of the brain if not treated.
• A study found that children who underwent a hemispherectomy often improved academically [and physically] once their seizures stopped.
• Hemispherectomy is one of the most drastic kinds of brain surgery and it is done only when not doing so will be worse.