We performed electrophysiologic studies of photoreceptor function in three patients with multiple evanescent white dot syndrome. During the acute stage, while the visual acuity was impaired, the electroretinogram (ERG) a-wave and the early receptor potential (ERP) amplitudes were profoundly decreased. The ERP regeneration times, determined for one subject, were prolonged. These findings suggest that photoreceptor function was impaired (abnormal a-wave), the effective visual pigment optical density of the outer segments was markedly reduced (ERP amplitude), and visual pigment regeneration was abnormal (ERP regeneration kinetics). During the recovery stage, the ERG and ERP amplitudes and visual acuity returned to normal. Our patients' disease seemed to be primarily of the retinal pigment epithelium (RPE). The decreased visual pigment density and prolonged regeneration kinetics emphasize the physiologic dependence of the sensory retina on the RPE.