Objectives: The purpose of this study was to evaluate differences in the severity of bronchiolitis in children younger than one year of age before and after the lockdown caused by the covid-19 pandemic. Materials and methods: This retrospective study examined children up to 365 days of age hospitalized for bronchiolitis in two periods, classified by before and after the lockdown: from 1st of April, 2018 until 31st of March, 2020 and from 1st of May, 2021 until 30th of April, 2023. Focus was placed on the influence of the lockdown on the severity of bronchiolitis in children younger than one year of age. We analyzed the following parameters: general information of the studied children (hospitalization rate of bronchiolitis, virus, coinfection, monthly admission, length of stay in hospital), socio-demographic data (sex, age) and the severity of bronchiolitis (bacterial infection, oxygen therapy, mechanical ventilation/respiratory support, intensive care unit). Results: Our survey consisted of 323 children comprising 163 before and 160 children after the lockdown. After screening for the diagnosis of bronchiolitis, our study included only 247 children who were used for the further major examinations. 128 children of them belonged to the period before the lockdown, the remaining 119 children participated in the period after the lockdown. RSV was the most common detectable viral agent in both phases with an increase in the post-lockdown period (72.7 % vs. 84.0 %; P = 0.021). Seasonal occurrence of bronchiolitis, specifically RSV-bronchiolitis, shifted due to the lockdown (P < 0.001). First cases were documented in the summer and early fall months with peak in November (0.0 % vs. 38.7 %). Few cases were recorded in January and February (21.1 % vs. 4.2 %, 39.8 % vs. 1.7 %). More boys were affected in both periods, but with a decreasing trend in the postlockdown phase (63.3 % vs. 55.5 %, P = 0.211). On average, studied children were younger after the lockdown (120 ± 87 days vs. 105 days ± 81 days, P = 0.249), with an increase in children younger than 29 days (10.2 % vs. 16.0 %). During both eras, the majority of patients studied were 29 days to less than three months of age (39.8 % vs. 37.0 %, P = 0.060). Bacterial superinfections were detected more frequently before the lockdown (7.0 % vs. 4.2 %, P = 0.337).The necessity of oxygen therapy was more apparent in the children after the lockdown (38.3 % vs. 48.7 %), even though without statistical significance (P = 0.097). The latter two parameters were independent of lockdown, sex and age of the patient. The occurrence of mechanical ventilation/respiratory support was more often after the lockdown (1.6 % vs. 6.7 %) and significantly associated with age (P = 0.010), but again without statistical significance (P = 0.053). Following the lockdown, more children required intensive care and treatment (6.2 % vs. 16.0 %, P = 0.014), significantly influenced by lockdown and age (P = 0.040, P = 0.003). Our study achieved no difference in the case of coinfection (1.6 % vs. 1.7 %) and length of hospital stay (two days most frequently, 4 days ± 2 days and 4 days ± 3 days on average). Conclusion: The introduction and subsequent enactment of the lockdown with its associated NPIs have an impact on the severity of bronchiolitis in pediatric patients. Upward trends concerning oxygen therapy, mechanical ventilation/respiratory support, and intensive care could be demonstrated. However, further studies are needed to investigate the severity of bronchiolitis in children under the influence of the lockdown in more detail, with greater precision and in a larger population.