Huntington's disease (HD) is a late-onset degenerative disorder of the central nervous system, caused by a dominantly inherited mutation in a gene on chromosome 4p. The identification of the trinucleotide repeat mutation responsible for this disorder has been an important step towards understanding the molecular pathology of HD, but in the meantime has also made it possible to offer predictive testing and prenatal diagnosis to individuals at high genetic risk. Predictive testing offers obvious benefits for those who receive a favourable result, but also runs the risk of significant psychological and social problems for the families involved. Uptake of testing to date has been limited. Prenatal testing where the pregnancy is at 25% risk carries the same disadvantages as adult predictive testing, because an unfavourable result would also establish that the at-risk parent is a gene carrier; prenatal exclusion testing offers an alternative method of detecting and terminating at-risk pregnancies without revealing the genetic status of the at-risk parent.