STUDY DESIGN: Prospective cross-sectional blinded-assessor cohort study. OBJECTIVE: The aim of this study was to determine the inter-rater reliability of the modified Japanese Orthopaedic Association (mJOA) score in a large cohort of degenerative cervical myelopathy (DCM) patients. SUMMARY OF BACKGROUND DATA: The mJOA score is widely accepted as the primary outcome measure in DCM; it has been utilized in clinical practice guidelines and directly influences treatment recommendations, but its reliability has not been established. METHODS: A refined version of the mJOA was administered to DCM patients by two or more blinded clinicians. Inter-rater reliability was measured using intraclass correlation coefficient (ICC), agreement, and mean difference for mJOA total score and subscores. Data were also analyzed with analysis of variance for differences by mJOA severity (mild: 15-17, moderate: 12-14, severe: <12), assessor, assessment order, previous surgery, age, and sex. RESULTS: One hundred fifty-four DCM patients underwent 322 mJOA assessments (183 paired assessments). ICC was 0.88 for total mJOA, 0.79 for upper extremity (UE) motor, 0.84 for lower extremity (LE) motor, 0.63 for UE sensation, and 0.78 for urinary function subscores. Paired assessments were identical across all four subscores in 25%. The mean difference in mJOA was 0.93 points between assessors, and this differed by severity (mild: 0.68, moderate: 1.24, severe: 0.87, P = 0.001). Differences of ≥ 2 points occurred in 19%. Disagreement between mild and moderate severity occurred in 12% of patients. Other variables did not demonstrate significant relationships with mJOA scores. CONCLUSION: The inter-rater reliability of total mJOA and its subscores is good, except for UE sensory function (moderate). However, the vast majority of assessments differed between observers, indicating that this measure should be interpreted carefully, particularly when near the threshold between severity categories, or when a patient is reassessed for deterioration. Further efforts to educate clinicians on administration and to refine the UE sensory subscore may enhance the reliability of this tool.Level of Evidence: 1.