To investigate systematically the natural history of visual outcome in branch retinal artery occlusion (BRAO). Cohort study. We included 199 consecutive untreated patients (212 eyes) with BRAO, first seen in our clinic from 1973 to 2000. At first visit, all patients had a detailed ophthalmic and medical history, and comprehensive ophthalmic evaluation. Visual evaluation was done by recording visual acuity, using the Snellen visual acuity chart, and visual fields with a Goldmann perimeter. The same ophthalmic evaluation was performed at each follow-up visit. Visual acuity and visual fields. We classified BRAO into permanent (133 eyes) and transient (18 eyes) and cilioretinal artery occlusion (CLRAO; 61 eyes). In eyes with permanent BRAO, of the 61 eyes seen within 7 days of onset, initial visual acuity was >or=20/40 in 74%, central scotoma in 20%, central inferior altitudinal defect in 13%, and inferior nasal and superior sector defects in 29% and 24%, respectively. Of those with follow-up, in the eyes with visual acuity or=20/40 and 1 (6%) or=20/40 in all during follow-up. In CLRAO on follow-up of 9 eyes, the central field improved in 4. When CLRAO was associated with retinal vein occlusion (38 eyes) or giant cell arteritis (12 eyes), visual findings were influenced by the associated diseases. These findings show that a visual acuity of >or=20/40 is seen initially in 74% of cases of permanent BRAO, 94% of transient BRAO, and 73% of nonarteritic CLRAO alone; and finally on follow-up, in 89%, 100%, and 100% of cases, respectively. The effectiveness of various treatment modalities for visual outcome has to be judged against this background. The authors have no proprietary or commercial interest in any materials discussed in this article.