Background.Influenza infections can cause severe respiratory disease in high risk persons such as those with asthma, but immunization rates for high risk groups remain suboptimal. An investigational influenza virus vaccine, trivalent, types A and B, live, cold-adapted (CAIV-T) administered by intranasal spray was shown previously to be effective in healthy adults and healthy children.Purpose.To assess the safety and tolerability of CAIV-T in subjects 9 years of age and older with moderate to severe asthma.Methods.In this randomized, double blind, placebo-controlled study, spirometry was performed twice before vaccination to establish a baseline forced expiratory volume at 1 s (FEV1) and once 2 to 5 days thereafter. The primary outcome index was the percent change in percent predicted FEV1before and after vaccination. Peak flows, clinical asthma symptom scores and nighttime awakening scores were measured daily from 7 days pre- to 28 days postvaccination.Results.The primary outcome index (percentage change in percent predicted FEV1) was not different between the two groups (0.2%vs.0.4% for the treatment and placebo groups, respectively;P= 0.78). Secondary outcomes did not differ between the two groups; these included the number of subjects with a decrease in FEV1≥15% from baseline, reductions in peak flows ≥15%, ≥30% or ≥2 sd below baseline, use of beta-adrenergic rescue medications, asthma exacerbations and clinical asthma symptom scores before and after vaccination. The same proportion of subjects in each group experienced postvaccination symptoms within 10 days (92% and 91%, respectively;P= 1.0). No serious adverse event occurred.Conclusion.CAIV-T was generally safe and well-tolerated in children and adolescents with moderate to severe asthma.