Design: A multicenter triple-blind randomized parallel-group, placebo-controlled trial of vitamin D3 supplementation. Children aged 1-5 years with (i) physician-diagnosed asthma, predominantly triggered by upper respiratory tract infections (URTIs), (ii) ≥4 reported URTIs in the past year, and (iii) ≥1 exacerbation requiring OCS (a recognised marker of moderate and severe exacerbations) in the past 6 months or ≥2 in the past 12 months, will be randomly allocated to one of two treatments in blocks of 4-6, stratified on recruitment site: Intervention group (n=432)—100,000 IU oral vitamin D3; control group (n=432)—identical placebo, for 2 oral doses, 3.5 months apart. Co-intervention with asthma therapy (preventive or pre-emptive ICS) will be left to the discretion of the physician and documented. Children will be followed every 3.5 months as per usual practice, with a home visit 10 days after each bolus, during which urine and blood will be sampled for urinary calcium:creatinine ratio, serum vitamin D, markers of calcium metabolism, and mechanistic exploration. A validated diary will serve to document the intensity and severity of exacerbations. In two sites, preschool lung function will be documented (if ≥ 3yrs). Outcomes: Primary endpoint-number of exacerbations requiring rescue oral corticosteroids (OCS) per child, documented by medical and pharmacy records. Secondary outcomes: duration and severity of exacerbations (symptoms & β2-agonist use, by diary; emergency visits, by medical records), parental functional status (by validated instrument), asthma therapy intensification and health care and direct costs (by health records & parent reports). Safety, mechanistic, exploratory outcomes: hypercalciuria, calcium metabolism, excess vitamin D, change in serum gene expression at 10 days (first 25 patients); and change from baseline at 7 months in preschool lung function (2 sites). Based on 3 published RCTs, a sample of 432 per arm (400+7.5% attrition) will provide 80% power (2-tailed alpha of 5%) to detect a 25% reduction in number of exacerbations requiring OCS/child (0.55 vs. 0.4125).