The authors report the clinical, neuroimaging, muscle biopsy and mtDNA findings in a patient affected by bilateral hearing loss and mental retardation since infancy, presenting at age 31 years with a rapid deterioration of mental status and ataxia leading to vegetative condition and death at the age of 32 years. Clinical and genetic studies have been also performed in the mother, affected by neurosensorial hearing loss. Muscle biopsy showed severe mitochondrial alterations in the propositus and evidence of mitochondrial alterations in his mother. Direct mtDNA sequencing in all family members revealed the known 7472insC mutation and the recently described A7472C sequence variation in the tRNA(Ser(UCN))gene. RFLP-PCR confirmed the heteroplasmic nature of the two mutations and failed to find the second transversion in 200 controls. The percentage of mutant genomes harbouring 7472insC ranged from 3 to 7% in asymptomatic family members to 70% in the proband and his mother, whereas the percentage of A7472C mutant genomes was about 90% in all maternal relatives except the proband (56%) and his sister (5%). In conclusion, this is the first report of a rapidly progressive encephalopathy in association with the 7472insC mutation in mtDNA, combined with an A>C variation at the same nucleotide with a possible suppression effect on the pathogenic mutation.