Report of a Joint Nordic Project with DACEHTA Denmark (closed in 2012), FinOHTA Finland, NOKC Norway, and SBU, Sweden. Obstructive sleep apnoea syndrome covaries with cardiovascular disease, including stroke and early death in men (moderately strong scientific evidence). There is insufficient evidence for women. There is insufficient scientific evidence of a relationship between obstructive sleep apnoea syndrome and arterial hypertension or diabetes mellitus. Obstructive sleep apnoea covaries with traffic accidents independent of daytime sleepiness and driving exposure among men (limited scientific evidence). The apnoea-hypopnoea index (AHI) shows good agreement between two nights of polysomnographic recordings (moderately strong scientific evidence). Manually scored portable devices including airflow, respiratory movements and pulse oximetry during one night of sleep have high sensitivity and specificity to identify a pathologic apnoea-hypopnoea index compared with polysomnography (strong scientific evidence). Automatic scoring of the results of portable devices has high sensitivity and identifies most patients with a pathologic apnoea-hypopnoea index, but specificity is low (strong scientific evidence). Automatic scoring programs cannot score sleep time and it is unclear whether these programs can differentiate obstructive from central apnoeas. Pulse oximetry with results from the oxygen desaturation index is insufficient to identify a pathologic apnoea-hypopnoea index and there is a high risk that patients with sleep apnoea syndrome will be incorrectly classified as normal (strong scientific evidence). A global impression from a case history and a physical examination alone are insufficient to identify or to rule out obstructive sleep apnoea syndrome (strong scientific evidence). There is strong evidence that CPAP reduces daytime sleepiness regardless of the severity of the sleep apnoea syndrome (strong scientific evidence). CPAP is highly effective in reducing obstructive sleep apnoeas (strong scientific evidence). There is contradictory scientific evidence concerning the effect of CPAP on quality of life (measured as functional outcomes and vitality) or arterial blood pressure. Tolerance and compliance with CPAP is good, and about 70% of patients still use it after 1–4 years for a mean of 5.3 (range 4.4–6.2) hours per night (moderately strong scientific evidence) – provided that patients and their CPAP equipment are seen by physicians shortly after treatment starts and subsequently at individual intervals, but always at least once a year. Mild to moderate discomfort from the CPAP mask – pain at the bridge of the nose, skin problems, air leaks and disturbing noise from the CPAP machine – are common adverse effects of CPAP (moderately strong scientific evidence). Mild nasal adverse effects, such as rhinitis, are common (limited scientific evidence). Auto-CPAP utilises a lower mean pressure than fixed CPAP, but the effects on daytime sleepiness, apnoea reduction and compliance are the same (strong scientific evidence). Custom-made mandibular repositioning appliances reduce daytime sleepiness in patients with mild to moderate sleep apnoea syndrome (limited scientific evidence). They reduce apnoea frequency but to a lesser extent than CPAP (limited scientific evidence). There is insufficient evidence concerning the effect of MRAs on quality of life (measured as functional outcomes and vitality) or arterial blood pressure. MRAs are still used by 76% of patients after one year and 56% after five years (limited scientific evidence). A majority of patients experience mild adverse effects – including discomfort in the teeth, hypersalivation and minor reductions in overjet and overbite – during the first few months (limited scientific evidence). There is insufficient scientific evidence for the effect of any surgical modality on daytime sleepiness or quality of life. There is contradictory scientific evidence for the effect of laser-assisted uvulopalatoplasty (LAUP) in reducing apnoea frequency. There is insufficient scientific evidence for other surgical interventions in reducing apnoea frequency. The adverse effects of uvulopalatopharyngoplasty (UPPP) due to snoring or obstructive sleep apnoea include serious perioperative and postoperative complications, including death, bleeding and respiratory compromise (moderately strong scientific evidence). Persistent adverse effects are frequent, and difficulty in swallowing occurs in about 28% of patients (moderately strong scientific evidence). Voice changes are also common (limited scientific evidence). The adverse effects of uvulopalatoplasty (UPP) and laser-assisted uvulopalatoplasty (LAUP) due to snoring or obstructive sleep apnoea include serious postoperative complications (limited scientific evidence). Persistent adverse effects occur in 50–60% of patients and difficulty swallowing in about 26% (moderately strong scientific evidence). Globus sensation in the throat and voice changes are common (limited scientific evidence). No studies that meet the present inclusion criteria show that weight reduction programmes, bariatric surgery, drugs, pacemakers, devices for sleep in lateral position, didgeridoo-playing or any other suggested treatment or lifestyle modification for obstructive sleep apnoea syndrome have any effect.