Background: The aim of this study was to determine the predictive risk factors for development of severe bronchiolitis in patients with acute bronchiolitis with no previous chronic disease. Methods: Four hundred forty children aged 1-24 months hospitalized with acute bronchiolitis, were examined between February 2018 and February 2019 in this prospective study. Results: Eighty-five cases were regarded as severe bronchiolitis and 355 as mild-moderate bronchiolitis. Statistically significant differences were observed between the severe and mild-moderate bronchiolitis groups in terms of weight-for-age z-scores, history of bronchiolitis, hemoglobin levels, and time elapsed between onset of symptoms and admission. Weight-for-age z-scores, the mean time interval between onset of symptoms and admission, and mean hemoglobin values were lower in the severe bronchiolitis group while the mean number of bronchiolitis attacks was higher than in the mild-moderate bronchiolitis group. Logistic regression analysis determined that a low weight-for-age z-score increased the risk of severe bronchiolitis development 0.56-fold (CI: 0.409 – 0.760), a short duration between the onset of symptoms and admission increased the risk 0.62-fold (CI: 0.519 – 0.735), a frequent history of bronchiolitis increased the risk 1.81-fold (CI: 1.135 – 2.968) and low hemoglobin levels increased the risk 0.72-fold (CI: 0.537 – 0.969). Conclusion: Low weight-for-age z-scores, a short duration between the onset of symptoms and admission, a high number of previous attacks, and low hemoglobin levels were identified as independent parameters of severe bronchiolitis development. Key words: Bronchiolitis, Infant, Risk factors, Severity degree