Objective: Acute bronchiolitis is one of the most common causes of hospitalization for children younger than 1 year. Although the prognosis for these children is generally good, the condition involves a risk of mortality. Here, we evaluate the immature platelet fraction (IPF) as a biomarker for the severity of acute bronchiolitis. Material and Methods: In total, 179 children who had been diagnosed with acute bronchiolitis were classified into three groups: mild, moderate, and severe bronchiolitis, and 80 healthy children were included as a control group. The diagnostic capacity of the IPF, mean platelet volume (MPV), platelet distribution width (PDW), white blood cell count (WBC), and platelet count (PLT) values to predict bronchiolitis was evaluated using receiver operating characteristic (ROC) curves and their respective areas under the curves (AUCs) calculated with 95% confidence intervals. Results: Patients with acute bronchiolitis had a larger IPF than their healthy counterparts (p < 0.001). Additionally, a positive correlation was observed between the clinical severity of the disease and the IPF. The ROC curve analysis indicated that the IPF cut-off point for predicting acute bronchiolitis was >3.2%, with a sensitivity of 84% and specificity of 97%. Our results clearly demonstrate that the AUCs for IPF, MPV, PDW, WBC, and PLT were statistically significant for the bronchiolitis versus the control group. The AUC was greatest for the IPF. Conclusion: The IPF is a new marker for diagnosing and evaluating the clinical severity of acute bronchiolitis.