* Age of onset of asthmatic symptoms: 0 - 18 years.
* Documented history of asthma diagnosed according to latest GINA guidelines, i.e. respiratory symptoms and either bronchodilator reversibility (improvement in FEV1 of more than 12% of baseline (and at least 200 mL) after inhalation of 800 µg salbutamol).
* Use of inhaled corticosteroids or either persistent symptoms of wheeze, cough, or dyspnea or regular use of β2 agonists at least once a week during the last 2 months.
* PC20 methacholine \< 8 mg/ml.
Group 2. Non-asthmatic controls
* No history of asthma.
* No use of inhaled corticosteroids or β2-agonists for a period longer than 1 month.
* No symptoms of wheeze, nocturnal dyspnea, or bronchial hyperresponsiveness.
Subjects must be able to adhere to the study visit schedule and other protocol requirements.
Upper respiratory tract infection (e.g. colds), within 6 weeks.
Serious acute infections (such as hepatitis, pneumonia or pyelonephritis) in the previous 3 months.
Signs or symptoms of severe, progressive or uncontrolled renal, hepatic, hematologic, endocrine, pulmonary, cardiac, neurologic or cerebral disease.
Malignancy within the past 5 years (except for squamous or basal cell carcinoma of the skin that has been treated with no evidence of recurrence).
Known recent substance abuse (drug or alcohol).
Females of childbearing potential without an efficient contraception unless they meet the following definition of post-menopausal: 12 months of natural (spontaneous) amenorrhea or 6 months of spontaneous amenorrhea with serum FSH \>40 mIU/mL or the use of one or more of the following acceptable methods of contraception:
