A new hypothesis is presented explaining the preventive effect of tobacco smoking found on noise induced hearing loss and on the frequency of Parkinson's disease. The hypothesis is based on the finding of a melanocyte stimulation of tobacco smoking in the human oral mucosa, resulting in a higher melanin content in the epithelial cells, and a higher frequency of visible oral melanin pigmentation--smoker's melanosis. The preventive influence of smoking found in the cochlea and substantia nigra may also be due to a higher melanin content and to the ability of melanin to strongly bind specific chemical agents for a long time. Melanin may in this way act as a scavenger against cell toxic factors in these organs.