Huntington’s disease (HD) is a serious neurodegenerative disorder, for which no effective treatment is available. It affects about 6,000 people in the UK and is inherited, so many more people know they are at risk of developing the condition. The gene for the disorder has been identified and it is possible for those at risk to take a test to see if they will develop HD in the future. Some results are difficult to interpret as they indicate that the person concerned may or may not develop the condition over the course of a lifetime: this is called reduced penetrance. This study aims to obtain anonymised details on the patients in the UK who have had such a result and determine the probability that an individual with such a result will develop the condition by a specific age. In future, this will improve the quality of information that can be given to those affected about the likelihood of developing the condition.