We can not predict which wheezing child younger than 3 years of age benefits from systemic glucocorticoid and which one does not. It is not known whether the differences in the efficacy are related to the differences in viral etiology, atopy, immunogical maturity or age of the patient. The study aims to answer the following questions: 1. What is the viral etiology of acute childhood expiratory wheezing? 2. What is the efficacy of prednisolone in relation to age, atopy and viral etiology in acute childhood wheezing? 3. Does prednisolone treatment increase risk for secundary bacterial infection in acute childhood expiratory wheezing? 4. What is the significance of inflammatory markers in predicting the efficacy of systemic steroid or patient outcome in acute childhood expiratory wheezing? Study will follow randomized, double blind, placebo-controlled parallel design. Study will start in Septemper 2000 and will be performed at the Department of Pediatrics, Turku University Hospital, Turku Finland. The study population will be 300 hospitalized wheezing children aged 3 months – 15 years. Investigational drug will be prednisolone, first dose 2 mg/kg, then 2 mg/kg/d/3 (max. 60 mg/vrk) p.o. for 3 d and comparative drug will be placebo tablet similar to investigational drug with the equal dosage. The primary outcome will be the time until ready for discharge. The study will provide new and important information for the diagnostics, treatment, disease outcome and prevention of acute childhood expiratory wheezing.