AIM: Study\'s objective was to identify risk factors associated with bronchiolitis severity. METHODS: A retrospective chart review of all children <2 years old diagnosed with bronchiolitis at the University Medical Centre Ljubljana between May 2014 and April 2015, who were treated as outpatients (paediatric emergency department, PED group) or as inpatients in the standard hospital setting (WARD group) or in the paediatric intensive care unit (PICU group). Detection of respiratory viruses in nasopharyngeal swab was accomplished by RT-PCR. Severity was assessed by Wang Respiratory Score and hospitalization longer than 24 hours. RESULTS: The study included 761 children. The three most frequently detected viruses were respiratory syncytial virus (RSV), human rhinovirus (hRV) and human bocavirus (hBoV) (57.5%, 272/473; 25.6%, 121/473; 18.4%, 87/473). Patient groups differed in Wang Respiratory Score for the severity of bronchiolitis (P < 0.001). No differences regarding the causative viruses were found. There was a lower proportion of children with the presence of more than one virus in PICU group compared to other two groups (P = 0.017). The three groups significantly differed in age, birthweight, comorbidities, bronchodilator treatment and antibiotic usage. However, multiple regression analysis revealed that younger age and the use of antibiotics were associated with bronchiolitis severity defined as hospitalization for >24 hours. CONCLUSIONS: Respiratory syncytial virus, hRV and hBoV were the most frequently detected viruses. The majority of patients admitted to the PICU had only one virus detected. Younger age and the use of antibiotics were associated with bronchiolitis severity.