Multiple occasional changes in critical flicker frequency (CFF) were studied in patients who recovered from optic neuritis. Twenty-five patients (31 affected eyes) with onset visual acuity less than 0.5 and who showed recovered visual acuity exceeding 1.0 were included in this study. Recovery stages were determined individually as follows: T1, initial onset stage; T2, intermediate stage; T3, recovered stage when visual acuity was 1.0 or better; and T4, final follow-up stage. CFF was determined using red, yellow and green illuminated targets in a compact CFF measuring device recently developed at our department. The rates of abnormality were 100% at stage T1 for all colors, and gradually decreased as the stage advanced from T2 to T4. However, the rates of abnormality continued to be high at 67% in stage T3 for the red target and 37% in stage T4 for the red target. The rates of abnormal interocular difference in CFF in 15 unilateral optic neuritis patients were 100% for all colors at stage T1 and decreased gradually with recovery. Slight but definite abnormality of CFF was also noted in the silent eyes of clinically unilateral optic neuritis patients. The rates of abnormal CFF more than 7% in all colors could be detected in T1, T3 and T4. These results indicate that CFF abnormality can be detected even at the stage of recovery in the pathologic eyes and the fellow eyes of optic neuritis patients. CFF was also shown to be a sensitive indicator for detecting visual dysfunction in patients with optic neuritis.