We described the clinical and neuroradiological findings together with a transcranial magnetic stimulation study in two patient with hemiparkinson-hemiatrophy syndrome (HP-HA). In both patients the neuroradiological findings (MRI) and the central motor conduction were normal whereas the functional imaging studies (SPECT) showed asymmetrical perfusion in the basal ganglia; the intracortical inhibition at short interstimulus intervals and the silent period duration in the motor cortex contralateral to hemiparkinsonism were significantly increased only in one of the patient which has a poor response to L-Dopa therapy. These studies suggest that intracortical or thalamo-cortical neuronal inhibition may be increased in HP-HA. The etiopathogenetic considerations, the diagnostic criteria and the prognostic value of our finding to evaluate the clinical evolution of parkinsonism are discussed in the context of current models of basal ganglia-thalamo-cortical connectivity. Transcranial magnetic stimulation will provide valuable information for the differential diagnosis of the parkinsonian disorders and may predict the efficacy of L-Dopa therapy.