Rationale: Patients with asthma exhibit variable response to inhaled corticosteroids (ICS). Vitamin D is hypothesized to exert effects on phenotype and glucocorticoid (GC) response in asthma. Objectives: To determine the effect of vitamin D levels on phenotype and glucocorticoid (GC) response in asthma. Methods: Nonsmoking adult asthmatics were enrolled in a study assessing the relationship between serum 25(OH)D (vitamin D) concentrations and lung function, airway hyperresponsiveness (AHR) and GC response as measured by dexamethasone (DEX)-induced expression of MAP kinase phosphatase-1 (MKP-1) by peripheral blood mononuclear cells (PBMCs). Results: 54 adult asthmatics (FEV₁ of 82.9±15.7% predicted (mean±SD), serum vitamin D levels of 28.1±10.2 ng/mL) were enrolled. Higher vitamin D levels were associated with greater lung function, with a 21.0±9.2 mL (mean±SE) increase in FEV₁ for each ng/mL increase in vitamin D (p=0.03, r=0.8). Participants with vitamin D insufficiency (<30 ng/mL) demonstrated increased AHR, with a PC₂₀ FEV₁ of 1.03±0.2 mg/mL versus 1.92±0.2 in those with vitamin D 30 ng/mL (p=0.01). In ICS-untreated participants DEX-induced MKP-1 expression increased with higher vitamin D levels, with a 0.05±0.02-fold increase (p = 0.02, r=0.5) in MKP-1 expression observed for each ng/mL increase in vitamin D, a finding which occurred in the absence of a significant increase in IL-10 expression. Conclusions: In asthma, reduced vitamin D levels are associated with impaired lung function, increased airway hyperresponsiveness and reduced glucocorticoid response, suggesting that supplementation of vitamin D levels in patients with asthma may improve multiple parameters of asthma severity and treatment response. Clinical Trial Registry Information: ID# NCT00495157, NCT00565266 and NCT00557180 registered at www.clinicaltrials.gov