Retinal vein thrombosis is most common in old people, and is often associated with systemic vascular disease. One of its rare systemic causes is protein S deficiency. A case of a 21-year-old woman with retinal vein prethrombosis associated only with protein S deficiency is described. She presented with acutely reduced visual acuity and a central scotoma in her left eye. Warfarin therapy was initiated, and complete improvement in ophthalmoscopic findings was subsequently observed. This case illustrates that protein S deficiency is a factor that should be considered in cases of retinal vein occlusion, particularly in young patients.122134-5