Trinucleotide repeat expansions are involved in an increasing number of neurodegenerative disorders. Eight disorders are caused by translated CAG expansions with sizes usually below 100-200 repeats. Expansions are observed in unrelated genes, and the threshold above which the disease becomes manifest varies according to the locus. There is a strong negative correlation between age at onset and the number of repeats. Direct molecular diagnosis, which is now possible, allows classification according to genotype, thereby multiplying the number of related disorders. Molecular analysis is also useful to diagnose disorders with variable and overlapping clinical features. Recent findings suggest that intranuclear inclusions are a characteristic of disorders with translated CAG expansions. Their formation might constitute an important step in the pathological process. Friedreich ataxia is the first disorder caused by a trinucleotide repeat expansion located within an intron. The clinical spectrum of the disease and its diagnostic criteria have been recently reevaluated in a large series of patients. Interestingly, Friedreich's ataxia is now thought to be associated with intramitochondrial iron accumulation. Frataxin, the protein that is mutated, might normally be responsible for mitochondrial iron homeostasis in tissues that are affected by the disease.