The Core Outcome Measures Index (COMI) is a reliable and valid instrument for assessing multidimensional outcome in spine surgery. The minimal clinically important score-difference (MCID) for improvement (MCID(imp)) was determined in one of the original research studies validating the instrument, but has never been confirmed in routine clinical practice. Further, the MCID for deterioration (MCID(det)) has never been investigated; indeed, this needs very large sample sizes to obtain sufficient cases with worsening. This study examined the MCIDs of the COMI in routine clinical practice. All patients undergoing surgery in our Spine Center since February 2004 were asked to complete the COMI before and 12 months after surgery. The COMI has one question each on back (neck) pain intensity, leg/buttock (arm/shoulder) pain intensity, function, symptom-specific well-being, general quality of life, work disability, and social disability, scored as a 0-10 index. At follow-up, patients also rated the global effectiveness of surgery, on a 5-point Likert scale. This was used as the external criterion ("anchor") in receiver operating characteristics (ROC) analyses to derive cut-off scores for individual improvement and deterioration. Twelve-month follow-up questionnaires were returned by 3,056 (92%) patients. The group mean COMI score change for patients declaring that the "operation helped" was a reduction of 3.1 points; the corresponding value for those whom it "did not help" was a reduction of 0.5 points. The group MCID(imp) was hence 2.6 points reduction; the corresponding group MCID(det) was 1.2 points increase (0.5 minus -0.7). The area under the ROC curve was 0.88 for MCID(imp) and 0.89 for MCID(det) (both P < 0.0001), indicating that the COMI had good discriminative ability. The cut-offs for individual improvement and deterioration, respectively, were > or =2.2 points decrease (sensitivity 81%, specificity 83%) and > or =0.3 points increase (sensitivity 83%, specificity 88%). The MCID(imp) score of 2.2 points was similar to that reported in the original study (2-3 points, depending on external criterion used). The MCID(det) suggested that the COMI is less responsive to deterioration than to improvement, a phenomenon also reported for other spine outcome instruments. This needs further investigation in even larger patient groups. The MCIDs provide essential information for both the planning (sample size) and interpretation of the results (clinical relevance) of future clinical studies using the COMI.