Huntington's disease (HD) is progressively invalidating and caused by a CAG expanded mutation. We tested the effect of the mutation length on the rate of progression in a cohort of 80 patients clinically followed-up and genetically characterized. Two patients presenting an infantile and aggressive HD form starting under 10 years had over 90 repeats; the other patients did not show any influence of the CAG expanded number on the rate of progression. In conclusion, the CAG expanded repeat affects the disease progression only at a very upper pathological range and in rare cases initiating very early in the life, while it does not seem to affect in any way the severity of the phenotype in most HD patients. Other factors affecting the motor symptom progression, other than the expanded repeats, therefore have to be investigated.