The present study included 80 Egyptians infants with acute bronchiolitis; age ranged from 2 to 22 month and consisted of 60 males and 20 females. In addition, 30 apparently healthy infants of matched age and sex were also included as controls. Besides taking history, thorough clinical examination and routine laboratorial investigations, arterial oxygen saturation, RSV tissue culture of nasopharyngeal lavage, indirect immnoflourescence for serum IgM against RSV, serum IL-4 and IFN-gamma were done. Results obtained showed that viral bronchiolitis is predominant in male around 6 month of age and in winter months. RSV was identified in 72.5% of cases. Level of serum IL-4 was significantly higher in RSV positive patients than either RSV negative patients or controls (P < 0.001). Furthermore, in RSV positive patients, IL-4 level was significantly higher in severe and fatal cases than in milder and surviving cases (P < 0.001). Serum IFN-gamma showed no significant difference between RSV positive and RSV negative patients in RSV positive cases it was significantly lower in more severe and fatal cases than milder and living ones (P < 0.05). The ratio IL-4/ IFN-gamma was significantly higher in RSV positive than RSV negative and controls (P < 0.001 and P < 0.05 respectively). Absolute eosinophilic count (AEC) and percent of oxygen saturation (SaO2%) showed significantly lower values in RSV positive patients than RSV negative (P < 0.001), but the latter showed significantly higher AEC than controls (P < 0.05). In RSV positive group, both parameters were significantly lower in more severe and fatal cases than milder and living ones (P < 0.001). In RSV positive group, significant positive correlations were found between disease severity and either serum IL-4 or IL-4 /IFN-gamma ratio (r = 0.789 and r = 0.823, P < 0.001 respectively) but disease severity was inversely correlated with either AEC or SaO2 % (r = -0.962 and r = -0.828, P < 0.001 respectively). RSV was identified as the major etiologic virus of bronchiolitis in young infants. Viral tissue cultures and indirect immunoflourescene are of equally diagnostic value but both may be needed. Most of the severe and fatal cases are in RSV positive group. Th2/Th1 imbalance reflected on IL-4/IFN-gamma ratio is more deranged in RSV bronchiolitis with Th2 predominance especially in more severe and fatal cases. These conclusions may be a rationale for the implementation of antiviral drugs in severe cases of bronchiolitis, however, further studies are needed to demonstrate the effect of antiviral drugs on cytokines levels during therapy.