Pendred syndrome is an autosomal recessive inherited disorder. Obligatory features are profound deafness in childhood and defective organic binding of iodine in the thyroid gland. Therefore, goiter is a common symptom. Hypoplasia of the cochlea is another feature. Recently, the gene for Pendred syndrome was identified. We describe a boy whose sensorineural hearing loss in both ears progressed rapidly from about 50 to 60 dB at the age of 3 years and 3 months to more than 100 dB at the age of 4 years and 4 months. This loss was preceded by a medical history of a progressive hearing loss. The progressive nature of the hearing loss motivated a search for the cause. Dysplasia of the cochlea and a widened vestibular aqueduct were found. The results of thyroid function tests were normal, but he had an elevated level of thyroglobulin. The diagnosis of Pendred syndrome was confirmed by the positive results of a potassium perchlorate test, indicating defective organic binding of iodine in the thyroid gland. It is possible that the widened vestibular aqueduct was responsible for the increase in the hearing impairment. Aside from the branchio-otorenal syndrome, Pendred syndrome is the only other known genetic disorder with a widened vestibular aqueduct. If a child has progressive sensorineural deafness and a widened vestibular aqueduct, it is important to consider a diagnosis of Pendred syndrome. A widened vestibular aqueduct may help to elucidate the pathophysiologic characteristics of hearing loss in these genetic types of deafness in childhood.