We studied the only two living affected individuals who are part of a previously reported kindred that expresses a hereditary parkinsonism-dementia syndrome with Lewy body pathology. The electrophysiological characteristics of the hyperkinetic movement disorders in these patients were examined to provide physiological insights into the clinical phenotype of this syndrome. Evaluation of both patients showed 7-9 Hz electromyographic discharges in upper extremity muscles during postural activation, and one patient showed a 4-5 Hz discharge pattern correlating to a rest tremor. Brief (<50 ms) myoclonic electromyographic discharges were seen in both patients, and a time-locked relationship to a focal cortical premovement electroencephalographic potential was elicited in one patient. Somatosensory evoked potentials were not enlarged and long latency reflexes were not enhanced. Electroencephalography was normal in one patient but showed pathologic slow frequencies in the other. The electrophysiological findings show evolution which correlates with an apparent characteristic evolution of hyperkinetic movement disorders that accompanies the severe progression of parkinsonism-dementia in this kindred. These results have implications for the future study of this and similar syndromes.