Two families with dominantly inherited low-frequency hearing loss are described. The two families are different in mode of transmission and in audiological findings. On the basis of these two differences, it is claimed that two types of inherited low-frequency hearing loss can be distinguished. One type is the dominantly inherited low-frequency hearing loss with fully penetrant abnormal genes. The other is also most probably due to an autosomal dominant gene, but with incomplete penetrance and probably represents a malformation in the middle ear combined with a defect in the apical part of the cochlear mechanism.