A study of mutation, biological fitness, and patterns of family building in Huntington's chorea has been carried out, based on a previously reported population study of the disorder in South Wales. No unequivocal new mutation was identified among 101 kindreds containing 418 affected persons, which supports the extreme rarity of mutation in this disorder. Increased values of fertility and fitness were found, both in relation to unaffected relatives and to the general population of the area. The proportion of unmarried persons and pattern of family building was comparable in affected and unaffected subjects, and no correlation with age at onset or mode of clinical presentation could be found.