We have studied 77 patients from 38 families with Huntington's disease (HD) analyzing the genes trinucleotide repeats (CAG)n; we have compared the results with those of 154 chromosomes from control population. Patients with HD range from 39 to 70 repeats while healthy patients have between 12 and 36 repeats. These results show the great feasibility of this genetic test. We do not have found a correlation between expansion's repeats and age of onset except for young people with the disease; in these cases we have found high level of expansions (more than 65) that could be a prognostic parameter of the disease.