Objectives: Sudden sensorineural hearing loss (SSNHL) is an emergency disease with undefined pathogenesis in the otolaryngology department. In our previous study, we found patients with SSNHL had lower serum concentration of Matrix metalloprotease 9 (MMP-9) than healthy controls, and the result was accordant with auto-immune diseases. This study aimed to reveal the correlation between changes in serum MMP-9 concentration following treatment with the outcomes of patients and to provide further evidence that immune disorder was the main pathogenesis of SSNHL. Design, setting, and participants: Fifty-two patients with SSNHL, hospitalized in The First Affiliated Hospital of Chongqing Medical University from March 2019 to August 2019, were enrolled. The serum concentration of MMP-9 was detected by enzyme-linked immunosorbent assay (ELISA). Main outcome measure: The mean concentration of MMP-9 before treatment was compared with the post-treatment concentration by the Mann-Whitney U test. The correlations between favorable outcomes of patients and clinical characteristics were measured with the Chi-squared test and binary multiple logistic regression analysis. Results: In treatment responders, mean serum concentration was elevated from 106.85±41.40ng/ml to 144.03±37.65 ng/ml following treatment (P＜0.001), while in non-responders it decreased from 132.09±59.21 ng/ml to 106.82±49.93 ng/ml (P=0.142). Changes in MMP-9 concentration was the only factor associated with favorable outcomes (P=0.008, OR=5.13, 95% CI: 1.53-17.28). Conclusions: Elevated MMP-9 concentration is a potential prognosis biomarker in patients with SSNHL. These findings are in line with auto-immune diseases and indicate immune disorder is mainly pathogenesis in SSNHL. Keywords Sudden sensorineural hearing loss; Matrix metalloprotease 9; Immune disorder; prognosis; Enzyme-linked immunosorbent assay