Studies have reported a correlation between noise-induced TTS & structural changes in the organ of Corti such as disarray of stereocilia, swelling of afferent nerve fibers & distortion of supporting-cell bodies. However, no pathological change has been found consistently, & the mechanism of TTS remains unclear. The present study sought to quantify pillar-cell buckling in cochleae fixed when TTS was present & in those fixed after recovery from TTS. Noise-exposed chinchilla cochleae were selected from our permanent collection using the following criteria: a) the exposure was a 4-kHz OBN at 57-86 dB SPL for 24-216 h; b) animals were 1-3-yr-old; c) 34 cochleae were fixed 0-d post-exposure & 12 were fixed after 20-30 d of recovery. Eight control cochleae were also evaluated. All cochleae had been fixed with 1% osmium tetroxide & embedded in plastic. After polymerization, the cochlear ducts were dissected into flat preparations & examined by phase-contrast microscopy. Grade 0 indicated no outer-pillar damage. Grade 1 indicated bowing of the outer-pillar bodies. Grades 2, 3 & 4 indicated that the outer pillars were slightly, moderately, or severely buckled, respectively. For each cochlea, total exposure energy [E = log2 (Pa2 seconds)] was calculated. E ranged from 5.11 (Group 1; 57 dB SPL, 48 h) to 16.92 (Group 6; 86 dB SPL, 216 h). In the apical half of the cochleae, there was little damage to outer pillars in all Groups. In the basal half of the cochleae, little pillar damage was present in Groups 1-3. Cochleae in Groups 4-6 sustained increasing pillar damage that was concentrated in the base. This damage was significantly different from controls at 78-93% distance from the apex for Group 4; 8-13%, 38%, 53% & 63-88% for Group 5; & 73-83% for Group 6. For the 20-30-d recovery cochleae, the average pillar grades differed significantly from controls for Group 5 at 38%, 53-58% & 78% & for Group 6 at 8% distance. Thus, high-frequency noise exposures that produce TTS also lead to outer-pillar damage, the degree of which increases with E. Recovery from TTS correlates with repair of outer-pillar damage. The data presented here support the hypothesis that outer-pillar damage is a mechanism for TTS.