Study Design: Reliability study. Objectives: The radiographic diagnosis of non-union is not standardized. Prior authors have suggested using a cutoff of <1 mm interspinous process motion (ISPM) on flexion-extension radiographs, but the ability of practicing surgeons to make these measurements reliably is not clear. Methods: 29 practicing spine surgeons measured ISPM on 19 levels of ACDF from 9 patients. Surgeons relied on these measurements to report on fusion status. Inter-observer correlation co-efficients (ICC), standard error (SEM) and the minimum detectable difference (MD) of these measurements were calculated. We screened for clerical errors by checking measurements more than one standard deviation from the group mean. Results: The ICC for ISPM was .76 (.64; .88) with a SEM of 1 mm and a MD of 2.76 mm. Agreement on fusion status was moderate, with an ICC of .6 (.44; .76). After screening for and removing clerical errors, the ICC improved to .82 (.71; .91), SEM improved to .83 mm, and MD improved to 2.29 mm. Six reviewers had an ICC >.9. The ICC from these high performing reviewers was .94 (.9; .97), SEM was .45 mm, and MD was 1.26 mm. Conclusions: The MD of 2.29 mm in our study group was not precise enough to support a cutoff of <1 mm ISPM as the sole measurement technique in screening for non-union after ACDF, and there was only moderate agreement amongst surgeons on fusion status based on dynamic radiographs. More stringent techniques are necessary to avoid mis-diagnosing non-union in clinical studies. Future studies should consider auditing measurements to identify clerical errors.