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Introduction Preschoolers have the highest rate of emergency visits and hospitalisations for asthma exacerbations of all age groups, with most triggered by upper respiratory tract infections (URTIs) and occurring in the fall or winter. Vitamin D insufficiency is highly prevalent in Canadian preschoolers with recurrent asthma exacerbations, particularly in winter. It is associated with more URTIs and, in patients with asthma, more oral corticosteroid (OCS) use. Although evidence suggests that vitamin D supplements significantly decrease URTIs and asthma exacerbations requiring OCS, there is insufficient data in preschoolers. This study aims to determine the impact of vitamin D 3 supplementation on exacerbations requiring OCS, in preschoolers with recurrent URTI-induced asthma exacerbations. Methods and analysis This is a phase III, randomised, triple-blind, placebo-controlled, parallel-group multicentre trial of vitamin D 3 supplementation in children aged 1-5 years, with asthma triggered by URTIs and a recent history of frequent URTIs and OCS use. Children (n=865) will be recruited in the fall and early winter and followed for 7 months. They will be randomised to either the (1) intervention: two oral boluses of 100 000 international unit (IU) vitamin D 3 (3.5 months apart) with 400 IU vitamin D 3 daily; or (2) control: identical placebo boluses with daily placebo. The primary outcome is the number of exacerbations requiring OCS per child, documented by medical and pharmacy records. Secondary outcomes include number of laboratory-confirmed viral URTIs, exacerbation duration and severity, parent functional status, healthcare use, treatment deintensification, cost and safety. Ethics and dissemination This study has received ethical approval from all sites. Results will be disseminated via international conferences and manuscripts targeting paediatricians and respirologists, and to families of asthmatic children via our Quebec parents-partners outreach programme. If proven effective, findings may markedly influence the management of URTI-induced asthma in high-morbidity preschoolers and could be directly implemented into practice with an update to clinical guidelines. Trial registration number NCT03365687.