In a case of infarction in the lower cervical spinal cord, F-waves were lost in the paralyzed hand muscles 48 hours after the onset, in spite of normal compound muscle action potentials (CMAP). Subsequently the amplitude of CMAP decreased markedly in size 14 days later, when needle EMG revealed acute denervation. At 11 days after the onset MRI demonstrated a linear lesion in the ventral portion of the lower cervical spinal cord suggesting ischemia. Thus, the loss of F-waves is useful in early diagnosis of the lower cervical spinal cord infarction, which reflects the decrease in the excitability of the anterior horn cells taking place soon after ischemic insult.