Several epidemiological studies have estimated the prevalence of obstructive sleep apnoea syndrome (OSAS). As many of those suffering from sleep apnoea may be asymptomatic, the occurrence of sleep apnoea may be underestimated. The epidemiology of self-reported snoring, sleep apnoea and OSAS, and their relationships with various symptoms, were evaluated in 1504 randomly selected males and females, aged 30, 40, 50 and 60 years. Nocturnal respiration was determined in 748 participants using inductive plethysmography. Habitual snoring was reported by 19.1% (9.2-24.2%, age 30-60 years) of the males and 7.9% (3.8-11.7%, age 30-60 years) of the females. Respiratory Distress Index (RDI) was calculated as the number of apnoeas and hypopnoeas per hour lasting longer than 10 s. RDI >/= 5 per hour was found in 10.9% (7.1-18.3%, age 30-60 years) of the males and in 6.3% (5.3-7.6%, age 30-60 years) of the females. Hypersomnia increased with the severity of sleep apnoea (P < 0.005) and was reported by 15.9% of those with RDI >/= 5. The prevalence of OSAS (hypersomnia and RDI >/= 5) was 0.9% in the females, 1.9% in the males, and in total 1.4% of all aged 30-60 years. The sensitivity was 70.8% and the specificity was 47.7% for self-reported snoring predicting RDI >/= 5. The following factors were associated with RDI >/= 5: age (P < 0.05), gender (P < 0.0001), BMI (P < 0.0001), tobacco (P < 0.02) and alcohol (P < 0.05) consumption. Snoring correlated with age (P < 0.02), gender (P < 0.001), BMI (P < 0.0001) and alcohol consumption (P < 0.05). We conclude that sleep apnoea is common and many of those with sleep apnoea are asymptomatic. Self-reported hypersomnia and snoring are not sensitive enough alone to identify those with sleep apnoea. Sufficient control of the questionnaire is thus essential in studies on snoring and the risk of cardiovascular diseases.