Diffuse unilateral subacute neuroretinitis (DUSN) is a clinical syndrome characterized early by visual loss, vitritis, papillitis, and recurrent crops of gray-white retinal lesions and later by progressive visual loss, optic atrophy, retinal vessel narrowing, and diffuse pigment epithelial degeneration. Evidence is presented that it is caused by a nematode that is probably not Toxocara canis; that at least two nematodes of different sizes are involved; that there are at least two endemic areas for the disease; that these areas are related to the size of the nematode; that the nematode may remain viable in the eye for three years or longer and cause progressive ocular damage; that thiabendazole and diethylcarbamazine citrate are ineffective therapeutically; and that photocoagulation is effective in destroying the nematode. Surgical excision of the nematode was attempted in two patients.