OBJECTIVE To determine the role of vascular endothelial growth factor (VEGF) in macular edema secondary to branch retinal vein occlusion (BRVO). METHODS Aqueous humor samples were collected from 52 eyes with macular edema secondary to BRVO before intravitreal drug injections and from 62 control eyes with cataract. VEGF was measured using an enzyme-linked immunosorbent assay. Fluorescein angiography showed capillary nonperfused areas (NPAs). Macular edema was evaluated by optical coherence tomography as the central retinal thickness. RESULTS The mean aqueous VEGF levels in eyes with BRVO and control eyes with cataract were, respectively, 290.5 pg/mL ± 294.9 pg/mL (range 81.9 pg/mL-1567.3 pg/mL) and 118.0 pg/mL ± 50.1 pg/mL (range 24.6 pg/mL-241.1 pg/mL), which differed significantly (P < 0.0001). The mean VEGF level in eyes with BRVO without apparent NPA was 171.4 pg/mL ± 52.5 pg/mL (range 90.9 pg/mL-299.9 pg/mL), which was significantly higher than controls (P = 0.001). VEGF levels were correlated positively with the size of NPA (P = 0.0002) but not with the central retinal thickness. CONCLUSIONS The aqueous VEGF concentration in patients with macular edema secondary to BRVO increased significantly and was correlated significantly with the size of NPA. Aqueous VEGF increased even in eyes without apparent NPA.