There is considerable interest in the association of snoring and health consequences that have been linked to more severe sleep-disordered breathing, including obstructive sleep apnoea syndrome. The goal of this investigation was to assess the independent association of heavy, habitual snoring and daytime sleepiness. For this, a cross-sectional, population based study of snoring, sleepiness and other factors was conducted using the Warsaw sample of the Multinational Monitoring of Trends and Determinants of Cardiovascular Diseases (MONICA) study, a population-based multicentre study of cardiovascular disease. The well-defined MONICA sample of middle-aged males and females also allowed estimation of age- and sex-specific prevalences of habitual snoring in Polish adults. Data on self-reported snoring frequency and loudness, and daytime sleepiness using the Epworth Sleepiness Scale (ESS) and other questions, were collected with a postal questionnaire. Seventy-nine per cent of the MONICA sample completed the questionnaire, yielding a total of 1,186 participants. Of the total sample, 27% of the females and 48% of the males reported habitual snoring ("often" or "always"). There was an independent association of habitual snoring and excessive daytime sleepiness (EDS), sleepiness that interfered with work and with increasing ESS scores. Habitual snorers were 5.8 and 3.1 times more likely to report EDS in active and passive situations, respectively, compared to nonsnorers (all p<0.01). It is concluded that habitual snorers, most of whom are probably unlikely to have frank sleep apnoea syndrome, are at substantial risk for daytime sleepiness. These findings add support to the hypothesis that simple snoring is not benign and underscores the need for further research on health outcomes associated with this prevalent condition.