PURPOSE: In the USA and Europe, hypovitaminosis D is associated with increased asthma severity, emergency department (ED) visit, and impaired pulmonary function in asthmatic patients. However, in tropical countries, data on the effect of vitamin D status on asthma is limited. This study evaluates the relationship between vitamin D status and the level of asthma control as well as other asthmatic parameters. METHODS: Asthmatic children were evaluated for serum 25-hydroxyvitamin D, pulmonary function tests, a skin prick test, and the level of asthma control. RESULTS: A total of 125 asthmatic children were recruited (boys, 66.4%). Their mean age+/-SD was 10.8+/-3.0 years. Vitamin D statuses were: deficiency (<20 ng/mL) in 19.2% of the patients, insufficiency (20-30 ng/mL) in 44.8%, and sufficiency (>30 ng/mL) in 36%. The vitamin D levels were 25.9+/-9.4 ng/mL in uncontrolled patients, 29.2+/-8.6 ng/mL in partly controlled patients, and 27.9+/-8.0 ng/mL in controlled patients (P>0.05). There were no significant differences in pulmonary function, asthma exacerbation, inhaled-corticosteroid (ICS) dose, anti-inflammatory drugs, or ED visit or hospitalization between different vitamin D statuses. Vitamin D deficiency patients were older and had a delayed onset of asthma than insufficiency or sufficiency patients. There was no significant correlation between serum vitamin D and pulmonary function/doses of ICS. CONCLUSIONS: High prevalences of vitamin D deficiency and insufficiency were found in asthmatic children in Thailand; however, there was no significant relationship between vitamin D status and the level of asthma control or other asthma parameters.