The study aim is in two phases. First phase: to evaluate the influence of vitamin D deficiency on asthma severity, degree of airway obstruction and frequency of asthma exacerbations. Second phase: to evaluate if in patients with vitamin D deficiency (25-OH vitamin D levels below 20 ng/ml), vitamin D supplementation decreases the number of disease exacerbations.

The first phase of the study consisted of a cross-sectional examination of a cohort of 119 asthmatic patients, who had at least one year regular follow-up in our respiratory clinic, and were not receiving any vitamin D supplementation.

In the second phase patients with vitamin D deficiency (25-OH D values below 20 ng/ml), were recruited in a one year longitudinal study, during which they received vitamin D supplementation. The study started and ended in late autumn-winter season when, at our latitude, sun exposure is lowest. Symptoms, spirometry and number of exacerbations were recorded during both study phases.

Inclusion Criteria:

• All the consecutive patients with bronchial asthma of any severity, who presented for a scheduled visit at our Respiratory Clinic during the period October 2012-March 2013 and who had at least one year regular follow-up every three months

Exclusion Criteria:

• Acute exacerbation in the last month and treatment with vitamins and dietary supplements in the last year.