Aminoglycoside induced deafness has been linked recently to a predisposing homoplasmic mutation in the 3' end of the small ribosomal RNA (rRNA) gene of the human mitochondria (1555 A-->G) that makes the mitochondrial rRNA structurally more similar to its bacterial counterpart. This mitochondrial DNA mutation was consistently found in families in which the susceptibility to develop ototoxic deafness was inherited through the maternal lineage. However, the 1555 A-->G mutation was rarely found in sporadic patients in China, where a significant proportion of the population has been exposed to aminoglycosides. To further characterize the mutations predisposing to aminoglycoside ototoxicity, we analysed the 12S rRNA gene in 35 Chinese sporadic patients without the 1555 A-->G mutation. Using single stranded conformational polymorphism (SSCP) analysis, heteroduplex (HD) analysis, sequencing, and allele specific oligonucleotide hybridization, we found three out of 35 sporadic patients with unique sequence changes in the 12S rRNA gene. Two of the patients had homoplasmic mutations. One patient displayed localized heteroplasmy around nt 961, with an absence of the thymidine at this position and different populations of mitochondrial DNA with varying numbers of inserted cytosines. The description of these putative susceptibility mutations, in particular the heteroplasmic mutation around nt 961, provides further support for the important role of the mitochondrial 12S rRNA in genetic predisposition to aminoglycoside induced ototoxic deafness.