Objective: To evaluate if hypoalbuminemia on admission predict disease severity in children with acute bronchiolitis (AB). Working hypothesis: Hypoalbuminemia is associated with worse outcome in infants with AB. Study design: Single-centre prospective cohort study. Patient-subject selection: Infants aged <12 month-old with AB. Methodology: Serum albumin levels were determined within the first 24 hours upon inclusion. The primary outcome was the need of pediatric intensive care unit (PICU) admission. Results: We enrolled 90 cases of AB. Serum albumin was independently associated with C-Reactive protein levels (CRP) (rs=-0,28; p=0.002). Fourteen (15.5%) cases required PICU admission. They presented lower serum albumin levels (3.7 (0.11) vs 4 (0.5) g/dl; p=0.034) regarding those patients without severe illness. In the multivariate logistic regression analysis, hypoalbuminemia was independently associated with a higher risk of severe illness (adjusted Odds Ratio 4.1 (1.2-85); p=0.032). The area under the ROC curve for serum albumin to predict adverse outcome was 0.70 (95% Confidence interval of 0.59-0.79). A cut-off point of 3.5 g/dl presented a sensitivity of 0.71, specificity of 0.68, positive predictive value of 0.29, and negative predictive value of 0.92. Conclusion: Low serum albumin levels at admission are significantly associated with higher PICU admission rates in infants with AB. The inflammatory response could play a key role in the occurrence of hypoalbuminemia in AB.