Huntington's disease or chorea (HD) is an incurable neurodegenerative genetic disorder that typically manifests itself first in middle age. It is the most common genetic cause of abnormal involuntary writhing movements called chorea. It is much less common in people of Asian or African descent than in people from Western Europe. The disease is caused by a mutation carried on an autosomal chromosome. It is inherited dominantly, such that each child of an affected parent has a 50% chance of inheriting the disease.

Physical symptoms of Huntington's disease can begin at any age from infancy to old age, but are usually first noticed in middle-age, between 35 and 44 years of age, often when affected individuals have had children. On rare occasions, when 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 disease is caused by a mutation in the Huntingtin gene, which normally provides the genetic code for a protein that is also called "huntingtin". The mutation results in a different form of the protein, which gradually damages specific areas of the brain, although the exact way it does this is not fully understood. Genetic testing, which has been possible since the discovery of the mutation, can be performed before the onset of symptoms in the relatives of an affected individual, as an antenatal test, and also on test-tube embryos, raising ethical debates. Genetic counseling has developed to aid individuals with their options 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. Although the disorder itself is not fatal, 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 often required in the later stages of the disease, but there are emerging treatments to relieve some of its symptoms.

Self-help support organizations, first founded in the 1960s and increasing in number, have been working to increase public awareness, to provide support for individuals and their families, and to promote research. These organizations were instrumental in finding the gene in 1993. Since that time there have been important discoveries every few years and understanding of the disease is improving. Current research directions include determining the exact mechanism of the disease, improving animal models to expedite research, clinical trials of pharmaceuticals to treat symptoms or slow the progression of the disease, and studying procedures such as stem cell therapy with the goal of repairing damage caused by the disease.