The relationship between the tonal masking level difference performance and the behavior of the "sensitized speech tests" according to Bocca and Calearo, were studied in subjects with normal hearing who were suffering from CNS diseases. The results have shown that pathological masking level difference values are present in all cases with pathological sensitized speech tests, and are related principally to the following two conditions: (1) the bulbo-pontine lesions and (2) the central hearing functional asymmetry. In subjects with normal hearing who have CNS diseases, pathological tonal masking level differences indicate the existence of central hearing pathways lesions.