With many epidemiologic studies made to establish the prevalence of sleep-disordered breathing (SDB) and obstructive sleep apnea syndrome (OSAS) in Western countries, no such data have been reported in Korea. The purpose of this study was to examine the prevalence of SDB and OSAS, and their related factors in Korean adults aged 40-69 years. Among the total of 5,020 participants at the baseline examination of the Korean Health and Genome Study, a random sample of 457 men and women was studied with employment of overnight full polysomnography to determine the prevalence of SDB and OSAS. The prevalence of SDB (apnea-hypopnea index > or = 5) was 27% and 16% in men and women, respectively. When OSAS was defined by an apnea-hypopnea index > or = 5 plus excessive daytime sleepiness, its prevalence was 4.5% in men and 3.2% in women. Logistic regression analyses showed that sex, body mass index, and hypertension were closely associated with the risk of SDB. Our findings show that SDB is a common problem in the Korean adult population. Understanding and treatment of SDB may be essential in terms of intervention to reduce the risk of related medical problems.