One prominent feature of Huntington disease (HD) is the variable age at which the characteristic neurological or psychiatric symptoms appear. Ages of manifestation varying from 4 to 65 years are found in a sample of 95 HD pedigrees compiled since 1968 from the Southeastern United States. Significant parent-child correlations of age of onset indicate consistency of onset age within nuclear families. However, an average intrafamily range of 9 years and an average intrapedigree range of 12 years reveal substantial variability of onset age within these groups. Of the nine cases of juvenile-onset HD identified in this sample, seven were of paternal descent. The preponderance of juvenile patients inheriting the HD gene from a father confirms similar findings from other studies. In addition, a trend toward earlier onset in all offspring of paternal transmission suggests that the juvenile-onset phenomenon is only the tail of a shift in the curve of onset ages for this group. A trend toward earlier onset in successive generations was noted. This 'anticipation' may reflect the finding that persons of early onset in prior generations are selectively nonreproductive as a result of manifestation of the disorder. By identifying familial factors influencing onset age of HD, it may be possible to more effectively evaluate environmental factors that influence the onset of the disorder.