Dissociated motor loss due to cervical spondylosis and disc herniation was evaluated in 10 patients who presented with left deltoid paresis in the absence of sensory deficits or myelopathy. All of these cases underwent cervical anterior decompression. Based on magnetic resonance imaging, computed tomography myelography, and computed tomography discography, patients were divided into two pathologic types: The first showed focal bony spur and disc herniation with axial cord rotation and nerve root compression, and the second demonstrated ventral cord flattening. Electrophysiologic studies included evoked spinal potentials, motor evoked potentials, and evoked muscle action potentials. Motor evoked potentials, recorded epidurally from the ventral aspect of the thecal sac and the nerve root within the anterior discectomy or vertebrectomy sites, proved clinically most useful. Combining the latest available neuroradiologic and electrophysiologic information, 4 types of neural injury associated with deltoid pareses were identified in the 10 patients. The first included isolated C5 nerve root lesions; the second, C6 nerve root lesions; the third, both C5 and C6 nerve root lesions, and finally, intrinsic cord pathology.