A person inheriting the HD gene usually develops signs of the illness very gradually, so that they may go unnoticed for years. The age at which an individual develops signs and symptoms is related to the size of the expansion of the HD gene, with larger expansions leading to an earlier age of onset. However, this relationship is very variable: it is not currently possible to predict accurately the age of onset from genetic test results or any other information.

HD causes a movement disorder, psychiatric difficulties, and cognitive changes, usually beginning in middle adult life. Signs and symptoms vary from person to person. For instance, one person affected with HD may have a very obvious movement disorder, but only mild psychiatric symptoms and intellectual deterioration, while another might suffer with depression and anxiety for years before showing any abnormal movements.

Cognitive disorder. HD causes difficulties in mental flexibility, so that it may become hard for the person with HD to switch quickly from one mental task to another. It may become difficult to learn new information or to remember things. However, a person with HD can usually recognize previously-learned information when it is presented to them. These cognitive changes can cause difficulties in the workplace and home before other symptoms of HD become evident.

Psychiatric disorder. Many of the psychiatric aspects of HD can be treated successfully. Depression is very common in HD, very treatable, and may precede the onset of other signs and symptoms of the illness. Severe depression may even lead to suicide attempts in a few individuals. Since depression is also common in the general population, it can be difficult to tell whether depression really represents the onset of HD in an at-risk person.

Other psychiatric difficulties seen in some people with HD include anxiety, obsessiveness, irritability, impulsiveness, social withdrawal, and trouble initiating activity. A few individuals with HD may have aggressive outbursts or even psychosis (hallucinations or delusions).