The activities of the auditory cortex of patients with severe sensorineural hearing loss were investigated by positron emission computed tomography (PET). The activity of the auditory cortex of a patient with residual hearing was nearly normal. In nearly deaf patients it was low, and the longer the duration of deaf period the lower was the activity. The activity of a prelingual deaf patient was low, not only in the primary auditory cortex but also in the secondary auditory cortex.