The prevalence of oral melanin pigmentation was studied in a population of 30 118 adult individuals in Sweden. Among these, 9.9% showed melanin pigmentation in the oral mucosa. The anterior labial gingiva in the mandible was the most frequently pigmented location. The presence of melanin pigmentation was positively correlated to tobacco smoking. Among tobacco smokers 21.5% were pigmented as compared to 3.0% among individuals not using tobacco. The smoking-related oral pigmentation, smokers' melanosis, could thus be calculated at a prevalence of 18.5% among smokers and the total frequency of smokers' melanosis was calculated at 6.8%. The prevalence of pigmentation was found to increase prominently during the first year of smoking but also to decrease to the level found among non-tobacco users about 3 years after cessation of smoking. Snuff dipping did not significantly elevate the prevalence of oral melanin pigmentation.