The large vestibular aqueduct as an isolated anomaly of the temporal bone has been previously identified radiologically, and its association with sensory neural hearing loss has been recognized. It has not, however, been defined as a distinct clinical entity in children. We studied 12 children, ages 3 to 9 years, with downward-fluctuating progressive high-frequency neurosensory hearing losses whose symptoms were thought to be related to the isolated enlargement of the vestibular aqueduct identified by high-resolution computed tomographic scanning. Previously it had been assumed that a large vestibular aqueduct is a temporal bone dysplasia that is a variant of the Mondini type of deformity and that the associated hearing loss is congenital in nature. Our clinical observations, however, indicate that the hearing loss in children with an isolated enlargement of the vestibular aqueduct is acquired during childhood. The natural history of this progressive deafness is reviewed, and a pathophysiologic hypothesis is presented.