Patients will be recruited from a variety of practice settings, including managed care organizations, community physicians, and academic centers.

Inclusion Criteria:

• Signed informed consent document (in the case of a minor, consent must be given by the child's parent or legally authorized representative)
• ≥12 years of age
• Physician diagnosis of moderate to severe persistent asthma
• Evidence or history of positive skin test or in vitro reactivity to an aeroallergen
• Willingness to participate fully for the duration of the study (5 years)
• For patients in the Xolair-treated cohort, have received at least one dose of Xolair therapy at the time of enrollment

Exclusion Criteria:

• Contraindication to Xolair therapy (e.g., patients who experienced a severe hypersensitivity reaction to Xolair)
• Acute asthma exacerbation within the previous 2 weeks of screening requiring any of the following: initiation of systemic corticosteroids, increased doses of systemic corticosteroids from baseline, doubling of inhaled corticosteroids, emergency room visit, or hospitalization
• Acute flare of significant systemic disease (e.g., infection, hematologic, renal, hepatic, cardiovascular diseases, or gastrointestinal diseases), or a recent hospitalization because of their disease within the previous 2 months
• Use of an experimental drug within 30 days prior to study screening
• Diagnosis of cystic fibrosis
• For patients in the non-Xolair-treated cohort, any prior treatment with Xolair