Retinal vein occlusion is an important cause of visual loss. Several ocular and systemic conditions have been reported for retinal vein occlusion. The pathogenesis of thrombus formation in the retinal vein, which results in retinal vein occlusion, is unclear. The aim of this study was to investigate the correlation between increased serum leptin levels and the occurrence of retinal vein occlusion (RVO). The study group consisted of 40 patients with RVO (58.1 ± 6 years old; 17 males and 23 females): 15 patients with central RVO, 23 with branch RVO, and 2 with hemispheric RVO. The patients who had any ocular or systemic pathology were not included in the study. The control group consisted of 40 healthy individuals of similar gender, age, date and type of health survey, and geographic region. The blood samples of the RVO patients (n = 40) and controls (n = 40) were obtained antecubitally. Leptin levels were measured by an enzyme-linked immunosorbent assay (ELISA) method, and Student's t-test was used to determine differences between the groups. The mean serum leptin levels were 12.5 ± 1.64 ng/ml in patients with RVO and 8.4 ± 1.22 ng/ml in the control subjects; namely, the mean serum leptin levels were significantly higher in the patients with RVO (p < 0.001). These results suggest that leptin may be involved in the pathogenesis of venous thrombosis in the retina probably through its effects on homeostasis of the vessel wall.