The objective of this study was to measure the impact of a 6-month delay in the diagnosis and treatment of patients with moderate obstructive sleep apnoea syndrome (OSAS) (apnoea/hypopnoea index (AHI) < 30) or severe OSAS (AHI > or = 30) on daytime sleepiness, cognitive functions, quality of life and healthcare expenditure (hospitalisations, medical visits, complementary tests, biological tests and drug prescriptions). In addition, this study aimed to analyse the incremental cost effectiveness ratios related to daytime sleepiness or quality of life following immediate introduction of treatment in these two populations. This study was conducted as a multicentre randomised controlled trial and carried out at two teaching hospitals in France. A total of 171 patients were followed for 6 months, with 82 patients randomised to group 1 "immediate polysomnography" and 89 in group 2 "polysomnography within 6 months". Patients with severe OSAS were deprived of a significant improvement of their daytime sleepiness (5.1 +/- 5.0 at the Epworth Sleepiness Scale score in group 1 versus 0.2 +/- 3.4 in group 2) and quality of life (12.4 +/- 13.3 at the Nottingham Health Profile score in group 1 versus 0.7 +/- 10.1 in group 2) during the waiting time. The impact of delayed management in subjects with less severe OSAS only concerned daytime sleepiness (1.1 +/- 3.3 in group 1 versus 0.3 +/- 4.3 in group 2). Delayed treatment did not affect cognitive functions or healthcare expenditure regardless of the severity of the disease. Incremental cost effectiveness ratios related to rapid introduction of treatment were significantly lower in the patients with more severe OSAS. These results provide fairly clear medical and economic arguments in favour of early management of patients with more severe forms of obstructive sleep apnoea syndrome.