BACKGROUND: Low vitamin D levels are common, and are associated with a higher incidence of future vascular events. We tested whether vitamin D supplementation could improve endothelial function and other markers of vascular function in patients with a history of myocardial infarction. METHODS: Parallel group, placebo-controlled, double-blind randomised trial. Patients with a history of myocardial infarction were randomised to receive 100,000 units of oral vitamin D3 or placebo at baseline, 2months and 4months. Outcomes were measured at baseline, 2 and 6months. Reactive hyperaemia index on fingertip plethysmography was the primary outcome. Secondary outcome measures included blood pressure, cholesterol, C-reactive protein, von Willebrand factor, tumour necrosis factor alpha, E-selectin, B-type natriuretic peptide, thrombomodulin and 25-hydroxyvitamin D levels. RESULTS: 75 patients were randomised, mean age 66years. 74/75 (99%) completed 6month follow-up. 25 hydroxyvitamin D levels increased in the intervention group relative to placebo (+13 vs +1nmol/L, p=0.04). There was no between-group difference in change in reactive hyperaemia index between baseline and 6months (-0.18 vs -0.07, p=0.40). Of the secondary outcomes, only C-reactive protein showed a significant decline in the intervention arm relative to placebo at 6months (-1.3 vs 2.0mg/L, p=0.03). Systolic blood pressure (+1.4 vs +2.3mmHg, p=0.79), diastolic blood pressure (+2.0 vs +0.8mmHg, p=0.54) and total cholesterol (+0.26 vs +0.24mmol/L, p=0.88) showed no between-group difference at 6months. CONCLUSIONS: Supplementation with vitamin D did not improve markers of vascular function in patients with a history of myocardial infarction.