The percentage of compliant continuous positive airway pressure (CPAP)-treated apnoeic patients that continue to experience residual excessive sleepiness (RES) is unknown. RES was defined by an Epworth Sleepiness Scale (ESS) score of >or=11. In total, 502 patients from 37 French sleep centres using CPAP >3 h night(-1) attending their 1-yr follow-up visit were eligible. ESS and polysomnographic data as well as symptoms, quality of life, depression scores and objective CPAP compliance at 1 yr were collected. Overall, 60 patients remained sleepy on CPAP (ESS 14.3+/-2.5) leading to a prevalence rate of RES of 12.0% (95% confidence interval (CI) 9.1-14.8). After having excluded associated restless leg syndrome, major depressive disorder and narcolepsy as confounding causes, the final prevalence rate of RES was 6.0% (95% CI 3.9-8.01). Patients with RES were younger and more sleepy at diagnosis. The relative risk of having RES was 5.3 (95% CI 1.6-22.1), when ESS before treatment was >or=11. Scores of emotional and energy Nottingham Health Profile domains were two times worse in patients with RES. As 230,000 obstructive sleep apnoea patients are currently treated in France by continuous positive airway pressure, more than 13,800 of them might suffer from residual excessive sleepiness.