Objective To assess the effect of continuous positive airway pressure treatment (CPAP) on subjective and objective sleepiness in patients with obstructive sleep apnea-hypopnea syndrom (OSAHS). Methods We conducted a thorough literature search to identify all published randomized controlled trials of CPAP in patients with OSAHS. We use computer to search Pubmed (1990-2008.5), CNKI (1994-2008.5), google (1995-2008.5), Springer Link (1997-2008)、 and many meeting articals. We chose the literatures that divided patients randomly into two groups as CPAP group and subtheraputic CPAP or drug group. Meta-analysis were performed to access the effect and bias. Results A total of 15 trials involving 1 052 patients were included. A Meta-analysis showed that (1) The total effect was that compared with the control group, CPAP treatment group significantly depressed the degree of sleepiness in ESS [WMD= – 2.61, 95%CI (– 3.92, – 1.29)] and in MWT/ MSLT [WMD= 1.42, 95%CI (– 0.01, 2.85)]. (2) Results of subgroups on ESS were that: 1. CPAP vs CT: there was a significant effect on CPAP therapy [WMD= – 3.15, 95%CI (– 4.84, – 1.47)]. 2. CPAP vs sham CPAP: there was no significant difference between the two groups. 3. CPAP vs drugs: CPAP was more efficient than drugs. CPAP may be more efficient in improving patients’ life quality. Conclusion CPAP is a valuable treatment for OSAHS no matter in subjective or objective sleepiness .But more attention should be paid to the lone-time treatment of CPAP.