Cases of juvenile Huntington's disease (JHD) represent the severe end of the phenotypic spectrum of HD. The first detailed description of JHD was written in German by Hoffmann in 1888. We have translated the article and report the description of two JHD cases in English; interestingly, in both cases the transmitting parent was the mother. The proportion of HD cases with juvenile onset varies between surveys, as summarised by Hayden in 1981. We have updated and extended this information based on 25 studies; the proportion of JHD cases ranges between 1% and 10.8% with a mean of 5.9% and approximately 1.4% having onset before 10 years of age. The prevalence of HD is around 70 per 1 000 000. The population of the countries that participate in REGISTRY is approximately 590 000 000 so there should be 2437 cases with JHD; in addition, there should be around 578 cases with onset before the age of 10 years. A problem with ascertainment may in part be related to delays in diagnosis. The Juvenile Huntington's Disease Working Group's aim is to carry out a more detailed study of the natural history of the condition and to develop more appropriate assessments for those with JHD, given their distinct phenotype and age. The recruitment of JHD patients in this global database will allow the development of rating scales to be used as outcome measures in clinical trials so that those patients with the most severe pathology can be included in studies of compounds which are considered to be neuroprotective.