Investigate the characterization of induced sputum cytology in preschool children with acute asthmatic attack and whether there is correlation between specific sputum cytology and response to therapy and to investigate airways hyper-responsiveness to adenosine 5'-monophosphate and to metacholine in pre school children 2-6 y old at 2 weeks and at 3 month following acute asthmatic exacerbation and look for correlation with response to treatment and sputum cytology. Clinical phenotypes of this patient population will also be investigated.