The characteristics of evoked potentials in patients with dentatorubral-pallidoluysian atrophy (DRPLA) were investigated. Twelve patients with DRPLA and three patients with progressive myoclonic epilepsy (PME) attributable to other causes participated in the study. In 11 out of the 12 patients, the diagnosis of DRPLA was genetically confirmed, based on a 56-75 CAG triplet repeat expansion on chromosome 12p; in the remaining patient, the diagnosis was not genetically confirmed but the patient was clinically diagnosed as having DRPLA and was within the same pedigree as one of the 11 genetically confirmed patients. Two out of the three patients with PME, who had been tested for dodecamer repeat expansion in the cystatin B gene, were genetically confirmed as having Unverricht-Lundborg disease (UL); the remaining patient was also clinically diagnosed as having UL, but the patient did not have the aforementioned genetic abnormality. Somatosensory evoked potentials (SEPs) and brainstem auditory evoked responses (BAERs) were recorded. The amplitudes of the SEPs were determined as the peak-to-peak amplitudes between P2 and N2 deflections. The results revealed that high-amplitude SEPs were not evoked in any of the DRPLA patients; on the other hand, high-amplitude SEPs were evoked in all the patients with UL. Moreover, BAERs were absent in seven out of the 12 patients with DRPLA; on the other hand, all UL patients showed BAERs in which all peaks, from I to V, were distinguishable. These results suggest differences in pathophysiology between DRPLA, which predominantly affects the brainstem and subcortical regions, and PME, characterized by cortical hyperexcitability. Thus, evoked potential measurements may be useful to differentiate DRPLA patients from those with progressive myoclonic epilepsy.