The impact of active smoking, passive smoking, and obesity on habitual snoring in the population is mainly unknown. We aimed to study the relationship of habitual snoring with active and passive tobacco smoking in a population-based sample. A total of 15,555 of 21,802 (71%) randomly selected men and women aged 25-54 years from Iceland, Estonia, Denmark, Norway, and Sweden answered a postal questionnaire. Habitual snoring, defined as loud and disturbing snoring at least 3 nights a week, was more prevalent among current smokers (24.0%, p < 0.0001) and ex-smokers (20.3%, p < 0.0001) than in never-smokers (13.7%). Snoring was also more prevalent in never-smokers exposed to passive smoking at home on a daily basis than in never-smokers without this exposure (19.8% vs. 13.3%, p < 0.0001). The frequency of habitual snoring increased with the amount of tobacco smoked. Active smoking and passive smoking were related to snoring, independent of obesity, sex, center, and age. Ever smoking accounted for 17.1% of the attributable risk of habitual snoring, obesity (body mass index > or = 30 kg/m(2)) for 4.3%, and passive smoking for 2.2%. Smoking, both current and ex-smoking, is a major contributor to habitual snoring in the general population. Passive smoking is a previously unrecognized risk factor for snoring among adults.