Background/aim: Huntington disease (HD) is a progressive adult-onset neurodegenerative disorder presenting an autosomal dominant inheritance. Since there is no information on the prevalence of HD in northwestern Iran, the aim of the present study was to determine the prevalence of HD and the number of CAG trinucleotide repeats in the population of northwestern Iran.Materials and methods: Genomic DNA was extracted from whole blood by the salting-out method. DNA samples were analyzed to determine the number of CAG trinucleotide repeats of the HD gene. An abnormally large number of CAG repeats, which is a diagnostic factor for the disease, was detected by polymerase chain reaction and agarose gel electrophoresis.Results: Out of 40 cases, we identified 14 nonkindred individuals with one expanded CAG allele at the IT15 gene. The frequency of the HD mutation in our group of patients was 35%. Expanded alleles varied from 36 to 70 CAG repeats, and normal alleles in HD patients varied from 20 to 26 CAG units. Conclusion: We found a significant correlation between age at onset of the disease and length of the expanded CAG tract: the lower the age, the longer the trinucleotide repeats length.