Background and aims: We aimed to analyze the correlation of urinary with serum NT-proBNP concentrations in acute bronchiolitis and its association with the severity of the disease.  Material and Methods: A pilot observational study conducted between 1st October and 31st March 2022, including acute bronchiolitis cases who attended our institution. Serum and urinary NT-proBNP concentrations were determined using the Alere NT-proBNP assay in time-matched urine and blood samples. We explored the linear relationship between both concentrations and compared clinical outcomes indicative of severe acute bronchiolitis between groups of raised and normal urinary NT-proBNP.  Results: 17 infants (median age 68 (36-91) days) with 36 time-matched samples were included. The urinary and serum concentrations of NT-proBNP were significantly correlated with (r=0.867 & R-squared coefficient=0.751; p<0.001). The log-10-transformed urinary NT-proBNP concentrations were higher at the time of hospital admission in those infants that required PICU admission with ventilatory support compared with those without this outcome (1.85 (1.16-2.44) pg/mg vs 0.63 (0.45-0.84) pg/mg); p<0.001); and resulted positively and strongly correlated with the duration of the ventilatory support (rho=0.76; p<0.001) and the LOS hospitalization (rho=0.84; p<0.001) Conclusion: The measurement of urinary NT-proBNP concentrations could be a reliable surrogate for serum NT-proBNP levels highlighting the potential value of the urinary NT-proBNP as a non-invasive tool to assess severity in acute bronchiolitis.