The rate of development of noise induced hearing loss (NIHL) in a group of 150 male workers exposed to industrial noise of over 85 dBA, was compared with the rate of a group of 40 unexposed workers. It was found that while the average annual rate of decline in the exposed group (for frequencies 1000-4000 HZ) was about 1 dB/year, it was less than 0.5 dB/ year in the unexposed group. These findings attest to the need to improve current prevention modalities (hearing conservation programs). In addition, the findings point to the importance of guaranteeing the accuracy of the audiometric tests as the usual "measurement error" is often greater than the expected real change (1-2 dB/year). These findings also question the value of current annual audiometry (a statutory requirement) in relation to providing significant new information.