Although health problems in musicians have been previously reported; not much is known about noise-induced hearing loss due to choir singing. However, there are data to show that peak levels of more than 110 dB SPL are produced in choir singing, and major parts of sound energy can be found below 1 kHz and even 500 Hz but not below 100 Hz. To find out about possible hearing loss due to professional choir singing, we measured the hearing threshold level of 62 choir singers in a large opera choir. Most publications about noise-induced hearing loss report that the high-frequency region is impaired most. However, in our study the low frequency region was affected most, when compared with normative data (especially ISO 7029). Control groups of women and men with normal auditory function did not show pure-tone hearing thresholds different from ISO 7029. The permanent threshold shifts at 250 Hz and above are most likely noise induced with choir singing as noise source. However, hearing losses at 125 Hz and possibly partial at 250 Hz are caused by some other effect. An (unproven) hypothesis is that singing might lead to increased endolymph pressure, and thus might cause hearing loss especially in the low-frequency region. Whether more choirs show similar hearing impairment and whether singing raises cerebrospinal fluid pressure will be the subject of further investigations.