The debate as to whether the sternomastoids receive ipsilateral, contralateral, or bilateral cortical innervation is based largely on the observation of stroke patients and, to a lesser extent, on animal experimentation. The variability of vascular lesions, the lack of pathology correlation in the early reports, and the differences in posture between humans and laboratory animals contributed to the controversy. We studied the function of the sternomastoid (SM) muscles during transient, complete left hemiplegia in 18 right-handed patients undergoing a Wada test. After injection in the right internal carotid artery (ICA), 14 patients were able to lift and turn their heads both to the right and to the left on command. Ten of the 14 patients who were able to follow commands after the injection had weakness of the right sternomastoid compared with the left. Our findings demonstrate that the left hemisphere can activate both the right and the left sternomastoid muscles during suppression of the right hemisphere. The sternomastoids receive bilateral hemispheric innervation and the maximal input comes from the ipsilateral hemisphere.