During 2 years of follow-up periods in 619 continuous positive airway pressure (CPAP) device users with obstructive sleep apnea syndrome (OSAS), we observed a 1.3% (8 patients) prevalence of residual sleepiness, defined as an Epworth Sleepiness Scale score of 11 or higher. We evaluated 12 of 98 cases with residual sleepiness among 1,161 CPAP users, using multiple sleep latency tests (MSLTs) on CPAP. Four patients were excluded because of CPAP suboptimal titration, or comorbid periodic limb movements. Another four patients demonstrated a mean sleep latency (SL) ≤8 min, and the remaining 4 patients demonstrated mean SL >8 min. Age was significantly higher in cases of mean SL>8 min than in cases of mean SL≤8 min. Cases in which objective sleepiness by MSLT could be proved required that recognized sleepiness be judged from the merger of hypersomnia, OSAS essential sleepiness, long sleepers, and healthy subjects. On the other hand, future examination is necessary in regard to handling that cannot prove objective sleepiness.