The reproductive performance of 157 patients with Huntington's disease, their 170 normal siblings and 150 matched unrelated controls is presented. Comparison with the general Canadian Prairie population in the appropriate age group is undertaken. Patients with Huntington's disease are not only more fecund than their unaffected sibs but also out-reproduce unrelated normal controls and members of the general population of corresponding age. Their fitness is thus estimated to be 1.14. Should the present reproductive pattern continue, the allele for Huntington's disease will double its frequency with consequent doubling of the prevalence of the disorder in six generations or approximately 150 years' time.