Bronchiectasis is a chronic, debilitating disease characterised by repeated respiratory infections. Evidence-based treatments are urgently needed. Vitamin D regulates the immune system and induces the secretion of chemicals that are capable of killing microorganisms. Vitamin D supplementation offers a potentially simple, safe and cost-effective treatment option for the prevention of infective exacerbations in patients with bronchiectasis. We plan to undertake a large randomised controlled trial of vitamin D supplementation. Several feasibility issues need to be assessed before this clinical trial can be undertaken. The aim of this feasibility study is to determine the range of serum vitamin D levels in patients with bronchiectasis, which are expected to be low, and assess whether vitamin D treatment will increase vitamin D levels adequately. The benefits of vitamin D supplementation are expected to include fewer exacerbations, improved quality of life, improved exercise capacity and lung function, and reduced acute healthcare utilisation and costs.