Pattern visual evoked response (PVER) and critical frequency of photic driving (CFPD) to repetitive flash stimuli were studied in 68 consecutive patients suspected of multiple sclerosis (MS) and 15 normal controls to assess the diagnostic value of combined PVER-CFPD testing. Clinically, 38 qualified for the diagnosis of MS while 30 did not. The CFPD was found to be unreliable for detecting optic nerve dysfunction (OND) in MS, since well-defined photic driving at all frequencies above 40 flashes per second was not present in 14 patients without MS (47%) and in three normal control subjects (20%). The PVER, on the other hand, was highly sensitive and specific for OND, being abnormal in 12 patients with definite MS (92%) but in none of the normal controls. It is concluded that PVER by itself is a valuable test for the diagnosis of OND in MS and additional CFPD testing is uninformative and may even be misleading.