Sleep-related breathing disturbances, especially obstructive sleep apnoea syndrome (OSAS), are commonly encountered. Epidemiological studies from different countries have shown that poor sleep and complaints about insufficient sleep or poor sleep are often related to poor health. Different studies are, however, difficult to compare with each other. One of the main reasons for this is the frequent methodological differences between questionnaires. There is a need for uniform methods: we need to know the prevalence and incidence of sleep disorders, judged using the same criteria, and also the severity of each problem, i.e. how often a problem or a symptom happens/occurs. In 1988 the Scandinavian Sleep Research Society formed a task group for developing a standardized questionnaire that could be used as a basis for questionnaires used in the Nordic countries. In this article we describe the Basic Nordic Sleep Questionnaire (BNSQ). The main change compared to many previous questionnaires is the five-point scale (scale from 1 to 5) stressing on how many nights/days per week something happens. The basic scale is: 1, 'never or less than once per month'; 2, 'less than once per week'; 3, 'on 1-2 nights per week'; 4, 'on 3-5 nights per week'; and 5, 'every night or almost every night'. For questions about specific rare events the first category may be divided into 'never' and 'less than once per month'. Habitually occurring events such as 'habitual snoring' are defined here as snoring every night or almost every night. The BNSQ has been used widely in a variety of studies performed in Nordic countries during the last years, and it has proven to be a valid tool.