The average asymmetry between the hearing threshold levels in the left and right ears was analyzed in a random population (n = 3487) representing a normal population. Males and females of age groups 5-10 years, 15-50 years and over 50 years were analyzed separately. A significant average inferiority of the hearing in the left ear was found at high frequencies, especially at 3-6 kHz, among adult males and females but not among children. A slight but statistically significant average superiority of the left ear at low frequencies was noted in all age groups. At corresponding hearing threshold levels at 4 kHz, the average inferiority of the left ear in the male population was significantly greater among subjects aged 15-50 years than among older subjects. In conclusion, the inferiority of hearing in the left ear at 4 kHz seems to be associated more with noise damage than with presbyacusis. The average interaural difference at 4 kHz was significantly more marked among subjects aged 15-50 years than among older subjects when analyzed at the corresponding threshold levels. These findings indicate a pathophysiological difference between noise damage and presbyacusis.