Six Italian families with familial nonsyndromic hearing loss consistent with a maternal pattern of inheritance were analyzed for mitochondrial mutations. The three known mitochondrial mutations associated with nonsyndromic hearing loss were investigated by polymerase chain reaction amplification, followed by restriction fragment length analysis or DNA sequencing. The A7445G mutation and C7472 insertion were not present in either of the families, but the A1555G mutation in the 12S rRNA gene was identified in homoplasmic form in two of the families. In one of the families the onset of hearing loss is congenital, while in the other it starts later in life. The families are from different regions of Italy, and mitochondrial haplotype analysis showed that the mutation arose independently in these two families. This suggests that the A1555G mutation may not be an uncommon cause of hearing loss in Italians, and is clinically important because maternal hearing relatives of patients with the A1555G mutation are at risk for aminoglycoside induced deafness. We discuss potential reasons for the normal phenotype in some relatives with the mutation, and the different onset of hearing loss in the two families.