Aim:Chiari malformation type 1 (CM1) has many symptoms due to tonsillar herniation through the foramen magnum. The assessment of CM1 surgery from the patient\'s point of view is scarce due to the paucity of patient-reported outcome measures (PROMs). Objective: We aimed to validate the Chicago Chiari Outcome Scale (CCOS) as a PROM tool and identify the cutoffs of CCOS-PROM and EuroQoL-5D-3L score index (EQ-SI) against Gestalt in adults who underwent CM1 treatment. Material and Methods:This cross-sectional study analyzed the health status of CM1 adults after a 12-month surgery follow-up. An electronic form was sent to groups of CM patients. The self-Gestalt good perception was distributed as a cure or improvement for validation purposes. AUROC was evaluated to verify the ability of each PROM to predict Gestalt outcomes. Ethical approval registry: 64148822.0.0000.5292.Results:The 85 subjects had a mean age of 42.1(±10.6) years and a follow-up of 4.8(±4.1) years. Gestalt outcome was good for 23(27%) patients. The median CCOS-PROM of 74 patients was 11(IQR 4). The median EQ-SI of 84 patients was 0.52(IQR 0.29). CCOS-PROM score of 13 was the optimal cutoff value (sensitivity 82.6%; specificity 78.4%). For EQ-SI, this value was 0.64 (sensitivity 82.6%; specificity 75.4%). The AUROC for CCOS-PROM and EQ-SI were 0.90 and 0.86, respectively.Conclusion:CCOS-PROM is valid for assessing the outcome of CM1 surgery in adults. CCOS-PROM score of 13 and EQ-SI of 0.64 were the best tradeoff values.