Friedreich's ataxia is an autosomal recessive disease generally characterized by the presence of microsatellite expansion in a GAA triplet. The patients inherit a pathologic allele from each one of their parents, that may sometimes show GAA triplet expansions or contractions. Two familial studies of typical Friedreich's ataxia are described. Their molecular study demonstrated marked intergenerational instability and an abnormally long expansion of the GAA triplet in the father in the other one. In the first case, there were expansions of 680/815 repetitions, being characterized by an expansion of 290 repetitions GAA in the father-patient transmission. The second case presented GAA of 1,260/1,095 expansions, while the expanded allele of the father was 1,350 repetitions. These cases illustrate that there can be both expansion as well as contraction of the GAA triplet from alleles of paternal origin in the disease, without any apparent phenotypic changes.