The etiology of otosclerosis is still unknown. Persistent measles virus infection of the otic capsule is supposed to be one of the etiologic factors in otosclerosis. The presence of measles virus was shown in otosclerotic patients by RT-PCR amplification of the viral RNA, detecting the viral proteins by immunohistochemistry and antimeasles immunoglobulin G in the perilymph samples. Nucleic acid (mRNA, vRNA, DNA) was extracted from pulverized, frozen stapes footplate samples of otosclerotic patients. Measles virus RNA was amplified by RT-PCR: reverse transcription and the first-round PCR amplification were performed by heat-stable recombinant Thermus thermophilus polymerase, while in the nested round PCR Taq polymerase was employed. Oligonucleotide primers specific to measles virus nucleoprotein and matrix protein RNA were used in these reactions. Edmonston- and Schwartze-type measles viruses served as positive controls and cortical bone fragments, stapes superstructures, cadaver stapes, incus and malleolar samples served as negative controls. Among 102 otosclerotic patients, 62 stapes footplate samples contained measles virus RNA. Measles virus RNA was not detected in other bone specimens of the patients. The etiologic role of measles virus in the pathogenesis of otosclerosis should be considered. The 40 negative samples may be genetically determined otosclerotic cases or stapes fixations due to other causes.