Two supposed measures of auditory frequency selectivity--the critical band (CB) in loudness summation and the psychoacoustic tuning curve (PTC), Both measured at 1 kHz--were compared with the capacity for speech discrimination in patients with various cochlear disorders and a relatively flat audiometric pattern. The CB in loudness summation was correlated neither to the degree of hearing loss nor to the speech discrimination score. In contrast, the PTC changed with increasing hearing loss in the same manner as the electrophysiological tuning curve (FTC), i.e. rapidly deteriorating beyond normal limit values when the hearing loss exceeded 30--40 dB (HL). Nearly the same dependency of the degree of the hearing loss was demonstrated for the speech discrimination score (determined in noise and after filtering of the signal) and a significant correlation was present between this score and cochlear tuning, as expressed by the PTC. It is proposed that the PTC is a more valid measure of auditory frequency selectivity than the CB in loudness summation. If this is accepted the results seem to support the hypothesis of impaired frequency selectivity as a major cause for deteriorated speech discrimination in patients with cochlear disorders.