In a study of 6500 workers in the shipbuilding industry, 8 subjects were found to have unilateral conductive hearing impairment established before the noise exposure period and without recurrent attacks of acute or chronic infection or clinical diagnosis of otosclerosis. All subjects demonstrated a more pronounced sensorineural hearing loss at 4.0 kHz in the ear with normal middle ear function. The results show the value of even a small permanent conductive hearing loss for protection against noise-induced hearing loss. The observations are discussed in relation to the role of individual variations in sound transmission, the value of the acoustic reflex in noise-induced hearing loss and the efficiency of continuous use of hearing protectors.