A 59-year-old patient with "exudative chorioretinitis" progressed from confluent infiltration of the deep layers of the sensory retina, perivascularly and at the level of the pigmentepithelium to total infiltration of the vitreous. This occurred in spite of all the conventional antiinflammatory therapy. A clinical diagnosis of primary reticulum-cellsarcoma of the retina was made because diagnostic vitrectomy for cytologic study was refused. Radiotherapy with 4000 R by Betatron led to a rapid disappearance of the infiltration in the vitreous and retina and to diffuse subretinal scaring of the entire fundus (histologically confirmed). One year later the patient died in a psychiatrically disoriented state. It is thought that the course after radiotherapy and the histological findings support, although not definitely confirm, the clinical diagnosis. Clinical criteria may suffice for the diagnosis in desperate situations to initiate effective radiotherapy in these fatally ill patients.