This is the first paper to study the physiological function of the spinothalamic tract in multiple sclerosis (MS) using pain-related somatosensory evoked potentials (pain SEPs) following CO2 laser stimulation. Among 12 patients with MS, hand- and foot-stimulated pain SEPs were significantly delayed or absent in 3 and 7, respectively. These results were totally consistent with clinical impairment of pain-temperature sensation. In contrast, the results of conventional electrically-stimulated SEPs were compatible with impairment of vibration sensation. Therefore, the examination of both pain and electric SEPs is very useful to evaluate the physiological function of the ascending spinal tract in patients with MS.