BACKGROUND: COPD is a common, preventable and treatable airway disease, and is currently the third leading cause of death worldwide. About one billion people worldwide are estimated to have vitamin D deficiency or insufficiency. Vitamin D deficiency is common among people with COPD, and has been reported to be associated with reduced lung function and increased risk of acute exacerbations of COPD. Several clinical trials of vitamin D to prevent acute exacerbations of chronic obstructive pulmonary disease (AECOPD) and improve COPD control have been conducted, but an up-to-date meta-analysis of all double-blind, randomised, placebo-controlled trials of this intervention is lacking. OBJECTIVES: To assess the effects of vitamin D for the management of acute exacerbations and symptoms for people with COPD. SEARCH METHODS: We searched the Cochrane Airways Trials Register and reference lists of articles. We also searched trial registries directly, and contacted the authors of studies in order to identify additional trials. The date of the last search was 24 August 2022. SELECTION CRITERIA: We included double-blind, randomised, placebo-controlled trials of vitamin D or its hydroxylated metabolites, for adults with a clinical diagnosis of chronic obstructive pulmonary disease based on the presence of characteristic symptoms and irreversible airflow obstruction. We did not impose restrictions regarding disease severity or baseline vitamin D status, in order to maximise generalisability. DATA COLLECTION AND ANALYSIS: We used standard Cochrane methods. The primary outcome was the rate of moderate or severe exacerbations (requiring systemic corticosteroids, antibiotics or both). We also performed subgroup analyses to determine whether the effect of vitamin D on the rate of moderate or severe exacerbations was modified by baseline vitamin D status, COPD severity or regular inhaled corticosteroid use. The main secondary outcomes of interest were the proportion of participants experiencing one or more exacerbations (moderate or severe), the change in forced expiratory volume in one second (FEV1, % predicted) and the proportion of participants with one or more serious adverse events of any cause, mortality (all-cause) and quality of life. We used GRADE to assess the certainty of evidence for each outcome. MAIN RESULTS: We included 10 double-blind, randomised, placebo-controlled trials in this review, involving a total of 1372 adults. Five studies contributed to the primary outcome analysis of the rate of moderate or severe exacerbations requiring systemic corticosteroids, antibiotics or both. The duration of studies ranged from six weeks to 40 months, and all investigated the effects of administering cholecalciferol (vitamin D AUTHORS' CONCLUSIONS: We found that administration of vitamin D results in little to no effect on the rate of moderate or severe exacerbations requiring systemic corticosteroids, antibiotics or both or the proportion of participants experiencing one or more exacerbations (moderate or severe) (both high-certainty evidence). Further, vitamin D probably has no effect on the inter-arm difference in change in lung volumes and the proportion of participants with one or more serious adverse event of any cause (both moderate-certainty evidence), and may make little to no difference to mortality or quality of life (both low-certainty evidence). We recommend further research on the balance of benefits and harms of vitamin D supplements in COPD for those with very low or very high starting vitamin D levels, because we assessed the available evidence as low-certainty for these groups.