 This study is a phase II, open-label study to investigate the bioactivity and pharmacodynamics of intravitreous ranibizumab in subjects with macular edema due to central and branch retinal vein occlusion. This pilot study will enroll 40 patients, 20 with central vein occlusion and 20 with branch vein occlusion. Each patient will receive three 3 injections of 0.5 or 0.3 mg of ranibizumab. The study consists of a 2-week screening period Days -14 to -1, a 3-month treatment period, and a 9-month follow-up period. Consented subjects will enter the 14-day screening period to determine eligibility, including serum chemistry and hematology testing, urinalysis, pregnancy testing, and macular thickness based on optical coherence tomography measurements and fluorescein angiography. Patients who have reduction of visual acuity to 2040 or worse due to foveal thickening from macular edema secondary to central or branch retinal vein occlusion and who meet eligibility criteria will be invited to enroll in the study. Baseline foveal thickness by OCT must be at least 250 um, which is often associated with VA of 2040 or worse and provides sufficient thickening so that a treatment effect is easily detectable Nguyen et al. 2004. Every effort will be made to recruit and enroll eligible patients from men and women of all ethnic and social backgrounds. It is expected that the 40 study subjects will be recruited over a 4-month period. Patients who meet entry criteria will be able to enroll in the study until the quota of patients has been achieved. All enrolled patients will receive either 0.5 or 0.3 mg injections of ranibizumab. Forty eligible subjects who have provided informed consent from one site Wilmer Eye Institute at the Johns Hopkins Medical Institutions will be enrolled, 20 with central vein occlusion and 20 with branch vein occlusion. In each of the 2 groups, 10 patients will be randomized to 0.5 mg of the ranibizumab, and 10 will be randomized to 0.3 mg of ranibizumab. Subjects will be identified and recruited through the clinic population of the Wilmer Eye Institute, including that of the Vitreoretinal Service and the Retinal Vascular Center, as well as through referral from physicians in the community. Announcement of the study will be made throughout the Wilmer Eye Institute, Johns Hopkins University School of Medicine through newsletters, pamphlets, and the clinical trials web site on the internet and intranet systems. Information about the study will also be sent to the community physicians as well as the ophthalmologists who often refer patients to the Wilmer Eye Institute. Every effort will be made to enroll eligible patients from men and women of all ethnic and social backgrounds. 