Corticomotoneuronal excitability was examined in 7 patients with dopa-nonresponsive progressive hemiparkinsonism (DNRHP) and 10 with dopa-responsive hemiparkinsonism (Parkinson's disease: PD), as well as in 10 normal subjects, by measuring change in motor evoked potentials (MEPs) using transcranial magnetic stimulation of the motor cortex after peripheral nerve stimulation. Corticobasal degeneration (CBD) was suspected clinically for the progressive dopa-nonresponsive hemiparkinsonism. Conditioning stimulation of the median nerve on the rigid side greatly increased MEP size (500-1300%) in 4 DNRHP patients as compared to the increase for the normal subjects (140-380%) at conditioning test (C-T) intervals of 40-80 ms, but stimulation on the contralateral side did not. There was no abnormal MEP facilitation on the rigid side in 10 PD patients. None of the patients with CBD had an increase in somatosensory evoked potential (SEP) size, and no patient showed hyperexcitability in SEP-recovery during C-T intervals at which there was abnormal MEP facilitation. These results indicate that there is an exaggerated effect of afferent input on corticomotoneuronal excitability in some patients with DNRHP.