Huntington's disease, chorea, or disorder (HD), is a progressive neurodegenerative genetic disorder, which affects muscle coordination and leads to cognitive decline and dementia. It typically becomes noticeable in middle age. HD is the most common genetic cause of abnormal involuntary writhing movements called chorea and is much more common in people of Western European descent than in those from Asia or Africa. The disease is caused by an autosomal dominant mutation on either of an individual's two copies of a gene called Huntingtin, which means any child of an affected parent has a 50% risk of inheriting the disease. In rare situations where both parents have an affected gene, or either parent has two affected copies, this risk is greatly increased. Physical symptoms of Huntington's disease can begin at any age from infancy to old age, but usually begin between 35 and 44 years of age. If symptoms begin before about 20 years of age, they progress faster and vary slightly, and the disease is classified as juvenile, akinetic-rigid or Westphal variant HD.

The Huntingtin gene normally provides the genetic information for a protein that is also called "Huntingtin". The mutation of the Huntingtin gene codes for a different form of the protein, whose presence results in gradual damage to specific areas of the brain. The exact way this happens is not fully understood. Genetic testing can be performed at any stage of development, even before the onset of symptoms. This raises several ethical debates regarding the age at which an individual is considered mature enough to choose testing, the right of parents to test their children, and confidentiality and disclosure of test results. Genetic counseling has developed to inform and aid individuals considering genetic testing and has become a model for other genetically dominant diseases.

The exact way HD affects an individual varies and can differ even between members of the same family, but the symptoms progress predictably for most individuals. The earliest symptoms are a general lack of coordination and an unsteady gait. As the disease advances, uncoordinated, jerky body movements become more apparent, along with a decline in mental abilities and behavioral and psychiatric problems. Physical abilities are gradually impeded until coordinated movement becomes very difficult, and mental abilities generally decline into dementia. Complications such as pneumonia, heart disease, and physical injury from falls reduce life expectancy to around twenty years after symptoms begin. There is no cure for HD, and full-time care is required in the later stages of the disease, but there are emerging treatments to relieve some of its symptoms.