The effect of clonazepam was studied in ten patients with nystagmus-induced oscillopsia due to downbeating or other primary position nystagmus. A 1-2-mg single-dose clonazepam test was used to determine whether long-term clonazepam therapy was indicated and to help distinguish between visual loss from underlying retinal or optic nerve disease and visual loss due to the nystagmus itself. With the single-dose clonazepam test, nystagmus was eliminated in 6 of 10 patients in the primary position of gaze and in 7 of 10 patients in downgaze. In all positions of gaze in all patients there was significant reduction in nystagmus intensity and slow phase velocity. Symptoms of oscillopsia were reduced or eliminated in all patients, and 7 of 8 patients with reduced visual acuity had clinical improvement. Guidelines are presented for the use of clonazepam in a single-dose clonazepam test and for long-term therapy.