Purpose: To investigate the serum heparanase concentration and heparanase activity of the patients with central retinal vein occlusion (CRVO) or branch retinal vein occlusion (BRVO). Methods: This prospective study included 23 CRVO patients, 13 BRVO patients and 28 control subjects. Serum heparanase concentration was measured by ELISA. Serum heparanase activity was determined using a heparan degrading enzyme assay kit (Mountain View, CA, USA). Multivariate logistic regression was used to adjust for the possible confounding factors when comparing the difference in serum heparanase concentration and activity between patients with RVO and control subjects. Results: Patients with CRVO (3.963 +/- 0.816 U/l) or BRVO(3.371 +/- 1.522 U/l) had significantly higher levels of heparanase protein compared to controls (1.055 +/- 0.902 U/l). This significance remained after adjusting for aforementioned confounding factors (CRVO versus control, p = 0.000, 95% CI: 2.4504.488; BRVO versus control, p = 0.006, 95% CI: 0.776-4.050). Patients with CRVO (0.3587 +/- 0.1498 U/ml) or BRVO (0.3616 +/- 0.0570 U/ml) had significantly higher levels of heparanase activity compared to controls (0.1449 +/- 0.0952 U/ml). The significance was maintained after adjusting for the aforementioned confounding factors (CRVO versus control, p = 0.012, 95% CI: 0.0360.275; BRVO versus control, p = 0.000, 95% CI: 0.150-0.395). There was no significant difference in serum heparanase protein levels and activities between CRVO and BRVO groups before and after confounding factor adjustment. Conclusion: Our study provides the first direct clinical link between systemic heparanase protein levels and heparanase activity with RVO, suggesting a critical role for heparanase in the pathophysiology of RVO.