Pattern reversal visual evoked potentials (VEP) and short-latency somatosensory evoked potentials (SEP) were recorded at frequent intevals in 7 patients with multiple sclerosis (MS). A persistent increase in latency of VEP was found to accompany evanescent visual symptoms. SEP, whether normal or abnormal, remained stable in patients without relapse. In 3 patients with active disease the cervical SEP was labile but this could not be related to specific clinical changes. In 51 patients VEP were recorded on two or more occasions at varying intervals. There was a reasonable correlation between alteration in visual acuity and in VEP. The return of abnormal latency to normal values was seen on 9 occasions. In 27 patients in whom SEP were similarly recorded the potentials became more abnormal in the majority of those with relapse involving the spinal cord but SEP did not return to normal with remission. It was concluded that the use of EP for diagnostic purposes in MS depended on persistence of abnormalities during quiescent phases of the disease and that this property was incompatible with the use of these techniques as an effective means of monitoring the course of the disease.