Female siblings with Pendred's syndrome were admitted to our clinic. The abnormality of the acoustic structure was examined by MRI. Bilateral enlargement of the vestibular aqueduct and a prominently marked endolymphatic sac were found on MRI. These findings seemed likely to represent a Mondini deformity. Acoustic structure in Pendred's syndrome was examined here by MRIfor the first time. Weexamined their HLA-DRlocus as a genetic marker using the affected sibpair method preliminary. HLAtyping might be a diagnostic criteria of Pendred's syndrome, although the present siblings possessed 2 HLAgenes in common.