Analysis of the polymorphic (CAG)n repeat in the huntingtin gene within the Chinese population in Taiwan confirmed the presence of an expanded repeat on all Huntington's disease (HD) chromosomes. Measurement of the specific CAG repeat sequence in 35 HD chromosomes from 11 unrelated families and 159 control chromosomes showed a range of from 9 to 29 (with a median of 17) repeats in normal subjects and 40 to 58 (with a median of 44) in affected subjects. The size distributions of normal and affected alleles did not overlap. The change in the size of the repeat in the HD size range on transmission for both sexes was variable. The expansion size inversely correlated with age at the onset in HD, especially from the early-onset disease. In summary, the molecular biology of HD is indistinguishable in Chinese and Caucasian populations, but the idea that world-wide HD evolved from a very limited number of European founders is no longer tenable.