The Huntington's disease gene contains an expanded unstable (CAG)n repeat, and the repeat lengths have been shown to correlate with the age of onset. Using detailed clinical scales, we evaluated the rate of progression of Huntington's disease and its relationship to the number of triplet repeats. We found significant positive correlation between the rate of progression of clinical symptoms (both neurological and psychiatric) and CAG repeat length. These data suggest an important role of expanded trinucleotide repeat length in affecting the pathological process during the entire course of Huntington's disease.