STUDY DESIGN: A retrospective comparison of magnetic resonance imaging (MRI) and quantitative electromyography (EMG) findings in patients with compressive cervical myelopathy (CCM). OBJECTIVES: To investigate which parameters of the EMG motor unit potentials (MUPs) as determined by automatic analysis of 4 muscles in the upper limb are correlated with spinal cord compression observed on MRI in CCM patients and to determine whether electrical and radiologic levels are correlated. SUMMARY OF BACKGROUND DATA: Increased mean duration of MUPs has been reported to be a sensitive indicator of disorders of the spinal motor neurons that are accompanied by axonal degeneration and regeneration. METHODS: MRI findings at each cervical disc level from C3/C4 to C6/C7 and MUPs recorded from 4 muscles (deltoid, biceps brachii, triceps brachii, and abductor digiti minimi) were examined to determine whether there is a statistical correlation between spinal cord compression and abnormal parameters of the MUPs for any combination of disc level and muscle. RESULTS: Significant correlations between increased mean duration of MUPs and radiologic level of cord compression were observed for deltoid and cord compression at C3/C4 (P < 0.01), biceps brachii and cord compression at C3/C4 (P < 0.001) and C4/C5 (P < 0.01), triceps brachii and cord compression at C5/C6 (P < 0.05), and abductor digiti minimi and cord compression at C6/C7 (P < 0.001). Other parameters of MUPs including amplitude, polyphasia and denervation potentials did not show significant correlation with compressive spinal cord lesions on MRI. CONCLUSION: Quantitative analysis of mean duration of MUPs provides a reliable indicator of physiologic disorder of spinal motor neurons in CCM and may contribute to establishing the site of motor neuron compromise in cases with multilevel spinal canal stenosis.