Twenty patients with presbycusis and a rather flat audiometric pattern had critical band estimation performed by the method of loudness summation, using noise bands centred around 1 kHz. The pooled data indicated a normal critical band both in patients with a hearing loss less than 50 dB HL and in patients with a hearing loss greater than or equal to 50 dB HL. The normal loudness difference between broad band and narrow band noise was reduced at all levels except the highest, giving rise to very flat CB functions. This reduction was more pronounced at the highest test levels in recruiting ears than in non-recruiting ears. A theory is suggested for parts of the anatomical and physiological correlate of the empirical phenomenon, the critical band.