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Background: In infants with bronchiolitis the quality and the quantity of the immune response may be related to the viral etiology and the development of respiratory sequelae. The Th1/Th2 pattern of the immune response during bronchiolitis remains unclear. Objective: The aim of our study was to understand the pattern of the immune response in infants with bronchiolitis from RSV or RV. Methods: Nasal washings from infants hospitalized with bronchiolitis were tested for 14 respiratory viruses and RSV and RV cases were enrolled. Demographic and clinical data were taken from the medical files. Peripheral whole blood was obtained immediately after diagnosis and stimulated with Staphilococcus enterotoxin B (SEB). Intracellular production of IFNγ and IL4 by CD4 T cells was analyzed by flow cytometry. Results: 31 RSV (16 boys, median age: 1.9 months, age range: 0.5-12 months) and 10 RV (5 boys, median age: 1.6 months, age range: 0.8-9.2 months) samples were analyzed. Overall, family history of atopy and asthma, demographic and clinical data did not correlate with IFNγ and IL4 production. Basal (p=0.012) and after SEB stimulation (p=0.003) IL4 production was higher in RV than in RSV infants. IL-4/IFNγ ratio indicated a Th2 pattern in infants with bronchiolitis from RV and a Th1 in infants with bronchiolitis from RSV. Conclusion: RSV and RV induce a different immunological pattern in young infants hospitalized for acute bronchiolitis. RV seems to predispose to future atopy, while RSV mainly induces a Th1 oriented immune response. According to our data, anti-inflammatory therapy might play a role in some infants with bronchiolitis.