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Background: Respiratory syncytial virus (RSV) is the most common cause of bronchiolitis, and in temperate climates, late fall and winter epidemics of bronchiolitis are usually linked to RSV. In recent years in Japan, the RSV infection epidemic has occurred earlier, even during the hot season. This study aimed to evaluate the seasonal variation of RSV-associated hospitalizations over the past decade and the relationship between season and disease severity., Methods: This was a retrospective single-center study. Hospitalized children were studied between 2011 and 2019. RSV was detected using rapid antigen detection tests. Clinical information was obtained from medical records, and patients were classified by 4 seasons of admission and analyzed for changes over time., Results: Among 3750 children, 945 (25.2%) were RSV-positive. The seasonal proportion of hospitalized children who are RSV-positive showed a peak shift towards summer, with a turning point in 2016. Comparing 2011-2012 and 2018-2019, incidence increased from 6.8% to 46.3% during summer, whereas during fall decreased from 50.0% to 20.7% and decreased from 28.4% to 20.7% during winter ( P < 0.05). A similar trend was observed in the number of children requiring oxygenation in the earlier period; however, after the transition, there was no significant difference between seasons., Conclusions: Our findings showed that the RSV epidemic shifted to peak during summer until 2016. There was an association between seasonality and severity, such that many younger children were hospitalized during the autumn and winter and required more oxygen; however, after 2016, this difference was no longer observed., Competing Interests: The authors have no funding or conflicts of interest to disclose., (Copyright © 2022 Wolters Kluwer Health, Inc. All rights reserved.)