We propose a randomized double blind multicenter clinical trial on infants 6 weeks to 12 months old with moderate or severe bronchiolitis, in 9 emergency departments of hospitals situated in different provinces across Canada, during 3 winter seasons. We hypothesise that infants with bronchiolitis treated with nebulized hypertonic 3% saline solution would have less risk of being hospitalized and would have shorter and less intense respiratory symptoms than those infants treated with nebulized normal saline solution. Our principal objective is to determine if nebulized 3% hypertonic saline solution reduces admission rate 48 hours after treatment compared to placebo. Secondary objectives are to compare between groups intensity of respiratory symptoms measured by different clinical scores (RDAI,PRAM, PASS and IRAS), duration of symptoms, length of hospital stay, added secondary effects and subsequent office visits for the same problem.