The study population will be 300 patients. To allow detection of a 18-h difference (half of the average duration of stay in our hospital) in the time until ready for discharge between the prednisolone-treated group and the placebo groups (with an estimated standard deviation of 24 h) and to maintain an alpha error of 0.05 and a beta error of 0.20, the required size of the sample is 27 children for each treatment group in RSV bronchiolitis (16). No estimate is available for rhinovirus group. If patients are separated to different groups according to e.g. age or viral infection, 108 patients are needed. On the other hand to get 31 patients with a viral infection (viral infection in 80 % of patients with expiratory wheezing, and sensitivity of methods 69 %), 49 children are needed in each treatment group (to assess different subgroups, 196 children) (9, 17). To take account the variations of epidemics, a total of 300 children will be recruited. The estimated time to get enough patients, which need hospitalization for expiratory wheezing, will be 2 years at the department of pediatrics, Turku University Central Hospital, Turku, Finland.