We meta-analytically summarized the associations of angiotensin converting enzyme (ACE) gene insertion/deletion (I/D) polymorphism with ACE activity and obstructive sleep apnea syndrome (OSAS) to see whether ACE activity is causally associated with OSAS. Literature search and data abstraction were done in duplicate. Sixteen articles including 2060 OSAS patients and 1878 controls were summarized. Overall, no significance was observed for the association of I/D polymorphism with OSAS, whereas carriers of II genotype (weighted mean difference or WMD, 95% confidence interval or CI, P: -11.976, -17.168 to -6.783, <0.001) or I allele (-9.842, -14.766 to -4.918, <0.001) had a lower level of serum ACE activity compared with DD genotype carriers, respectively. In subgroup analyses, carriers of II genotype were 3.806 times more likely to develop OSAS (95% CI, P: 1.865 to 7.765, <0.001) in OSAS patients with hypertension, without heterogeneity. Mendelian randomization analysis indicated there was 37.4% (95% CI: 1.115 to 3.142) and 32.4% (1.106 to 2.845) increased risk of OSAS by a reduction of 1 U/L in ACE activity for the II genotype and I allele carriers versus DD genotype carriers, respectively. There was no observable publication bias. Collectively, genetically-reduced serum ACE activity might be a causal risk factor for OSAS.