The objective of the study was to evaluate the relation between every-night (habitual) snoring, sleep apnoea and cognitive complaints (concentration and memory complaints) in an adult population-based sample. In the DAN-MONICA (MONItoring trends in CArdiovascular iseases) 1504 males and females aged 30, 40, 50 and 60 years were classified according to their snoring habits. Nocturnal respiration was measured in 748 participants. The following measures were regarded as potential confounders: age, gender, unintended sleepiness, insomnia, depression, hypnotic use, alcohol consumption by questionnaire, body mass index (BMI) and blood pressure. Concentration and memory problems were both related to depression, insomnia and unintended sleepiness. Snoring and sleep apnoea (defined as a respiratory distress index-RDI > or = 5) were associated with concentration problems and unintended sleepiness. The odds ratios (95% confidence intervals) between snoring, concentration and memory problems, calculated by logistic regression analysis after adjustments for the above confounders, were 1.90 (1.23-2.91, p < 0.01) and 1.38 (0.97-1.99, NS). For those with sleep apnoea, the odds ratios were 3.53 (1.42-8.73. p < 0.001) and 1.51 (0.81-2.14, NS) for concentration and memory problems, respectively. The main conclusion drawn from this study is that cognitive complaints show a high correlation to mood, insomnia and hypersomnia. Habitual snoring and sleep apnoea show a correlation to concentration problems, but not to memory problems. This suggests that part of the association between snoring, sleep apnoea and cognitive dysfunction is related to the presence of sleep disturbances and daytime sleepiness.