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Background: There are currently few reports on the etiology and the prognosis of C5 palsy using electrophysiological examination. The aim of this study was to discuss the etiology and prognosis of C5 palsy after anterior decompression with spinal fusion (ASF) using radiological findings and electrophysiological examination. Methods: 219 patients underwent ASF for cervical degenerative disease. We assessed the cervical sagittal alignment, the local angle at the fused level, and the height of the fused vertebral body on lateral radiographs in a neutral position preoperatively and at final follow-up. We performed intraoperative motor evoked potentials (MEPs) from deltoid and biceps, and measured compound muscle action potentials (CMAPs) in deltoid and central motor conduction time (CMCT) preoperatively and approximately 1 month after onset of the C5 palsy. C5 palsy was defined as a paresis of the deltoid (manual muscle testing (MMT) score of 1 or 2) with involvement of the biceps brachii muscle and supination. To compare the radiological and neurological findings of patients with C5 palsy and those without C5 palsy, 40 patients (designated as group C) were randomly selected from 209 patients without C5 palsy. Results: The incidence of C5 palsy was 4.6% (10 patients). We calculated the CMCT pre- and postoperatively in 8 patients. Compared with preoperatively, the CMCT shortened in 7 patients (87.5%) postoperatively, except for case 6. Intraoperatively, there were no patients with a decrease in MEPs from deltoid and biceps. Patients had a CMAP amplitude that exceeded 1.5 mV for the deltoid on the C5 palsy side recovered deltoid function. There were no significance differences in radiological findings between the group with C5 palsy and group C. Conclusions: We considered the prognosis to be good in patients with a CMAP amplitude of the deltoid muscle on the C5 palsy side that exceeded 1.5 mV.