We investigated a functional hemispheric asymmetry of the human motor system between right- and left-handed subjects using motor evoked potentials (MEPs) in first dorsal interosseous (FDI) produced by transcranial magnetic stimulation (TMS). TMS was applied while the subject imaged flexing the index finger, the mechanical action that maximally potentiates MEPs in the FDI. In right-handers, MEP amplitudes induced by motor imagery of right (dominant hand) finger movement were significantly larger than those induced by mental imagery of left (non-dominant hand) finger movement. In left-handed individuals, however, these right-left differences in MEP amplitude were not observed. Thus, these differences in the degree of hemispheric asymmetry in right- and left-handed individuals might reflect corresponding differences in their cerebral organization.