Objective Women with polycystic ovary syndrome (PCOS) demonstrate an increased prevalence of cardiovascular disease (CVD) risk factors. Previous researchers have compared flow-mediated dilation (FMD), an early marker of CVD, in women with and without PCOS. Evidence for a PCOS-mediated reduction in FMD remains equivocal, potentially because of study differences in cohort-matching and measurement approaches. The aims of this systematic review and meta-analysis were to examine to what extent FMD is impaired in PCOS and to explore the influence of potential moderators of FMD reduction, such as age and BMI. Design A systematic review and meta-analysis of published observational studies comparing FMD in PCOS with control women. Patients Twenty-one published studies were included (PCOS, n = 908; controls, n = 566). A subanalysis, using tighter inclusion criteria, involved seven studies (PCOS, n = 402; control, n = 251). Measurements Mean differences in FMD between PCOS and controls were synthesized. The subanalysis was delimited to the inclusion of age and BMI-matched controls. These factors were then explored as moderators using meta-regression. Results The pooled mean FMD was 3·4% (95% CI=1·9, 4·9) lower in PCOS compared with control women, with substantial heterogeneity between studies. In the subanalysis, the PCOS-mediated reduction in FMD was 4·1% (95% CI=2·7, 5·5). Heterogeneity remained substantial (I 2=81%). Subsequent meta-regression indicated that the magnitude of FMD difference was not influenced by BMI (P = 0·17) nor age (P = 0·38). Conclusions This systematic research synthesis indicates that endothelial function is compromised in PCOS women, even if they are young and nonobese.