Recessive mutations of PDS gene are the common causes of Pendred syndrome and non-syndromic hearing loss associated with temporal bone abnormalities ranging from isolated enlargement of the vestibular aqueduct (EVA) to Mondini dysplasia. In this study we evaluate the relationship between EVA and Mondini dysplasia in 10 prelingual deaf patients and PDS gene mutation. One of three mutations, IVS7-2A-->G, IVS16-6G-->A or IVS15+5G-->A, was identified in the PDS gene in each patient. In family studies of four probands with the IVS7-2A-->G mutation, we found that this mutation was inherited from the same mutant alleles of parental origin. The effect of IVS7-2A-->G mutation on PDS gene expression was determined by reverse transcription and polymerase chain reaction (RT-PCR). Sequencing of the RT-PCR products revealed that the PDS transcripts from the allele with IVS7-2A-->G mutation lose the entire exon 8, resulting in a joining of exons 7 and 9. Deletion of the exon 8 results in frameshift and premature termination of translation. Haplotype analysis showed a significant haplotype shared among the family members carrying IVS7-2A-->G mutation, suggesting that they may be derived from a common ancestor. Our results provide evidence that hearing loss with EVA and Mondini dysplasia may be caused by splice-site mutation in the PDS gene.