We present a left-sided hemiplegic patient with a cerebrovascular lesion involving the medial part of the right frontal and parietal lobes and the corpus callosum, but sparing the hand area of right primary motor cortex (M1). Several studies using transcranial magnetic stimulation demonstrated functional integrity of the efferent pathways from the right M1, intact sensory afferents to M1, an impairment of transcallosal connection between the bilateral motor cortices, and reduced ipsilateral cortico-cortical inhibition within the right M1. Based on these results, we conclude that the paralysis of this patient was caused by disconnection of the intact M1 from any structures requisite for initiation of movements. The present patient also suggests the importance of various afferents to M1 in voluntary movement. We propose a term of 'primary motor cortex isolation' to designate the paralysis reported here.