A diagnosis of severe stridor due to a subglottic tracheal mass was made in a 62-year-old woman. The tumor was removed by external transtracheal surgery, and the pathologic study disclosed that it was intratracheal ectopic thyroid tissue. In this particular case, 10 years prior to the onset of the stridor, normal thyroid tissue was seen in biopsy specimens of the same subglottic localization. Although the pathogenesis of a mass composed of intratracheal thyroid tissue is not known, this case shows that it can be a slowly progressive tumor that is asymptomatic for several years.