There has been no epidemiological study of snoring in Japan, and we therefore performed a questionnaire survey (in about 7,000 adult men working at a steel-making factory at the time of the yearly health examination, and investigated the relationship between the severity of snoring and 17 items including age, obesity, family history of snoring, daytime hypersomnolence, hypertension, smoking, alcohol intake and traffic accidents. We classified all the subjects into three groups, no snoring, mild snoring, and severe snoring group. We defined severe snorers as persons who snored loudly in both inspiratory and expiratory phases and those who snored loudly with apnea. We found that aging, obesity, smoking and alcohol intake are risk factors for snoring. Compared with non-snorers, severe snorers were found to have a high incidence of family history of snoring, daytime hypersomnolence, and history of treatment of hypertension. No relationship was found between the severity of snoring and the occurrence of automobile accidents. The proportion of severe snorers over 40 years old with obesity, daytime hypersomnolence and morning headache was 0.25%, representing the group that may have obstructive sleep apnea syndrome. The probable incidence of sleep apnea syndrome in men may be considerably lower in Japan compared with that in either U.S.A. or Europe.