Background. Chronic foot ulcers are a major cause of morbidity in people with diabetes with a lifetime risk of 25%. Treatment is challenging and the recurrence rates of foot ulcers are >50% after 3 years. Vitamin D deficiency is more common in people with diabetes with chronic foot ulcers, compared to both people without diabetes as well as people with diabetes but without foot ulcers. Purpose/aim of study. To assess the efficacy of high-dose compared to low-dose Cholecalciferol vitamin D3 on healing of chronic diabetic foot ulcers. Materials and methods. We included people with dia- betes with one or more foot ulcers lasting for more than 6 weeks. Patients were randomly allocated to either a daily oral intake of high-dose (170 μg) or low-dose (20 μg) vitamin D3 (Cholecalciferol). We saw patients in the outpatient clinic after 4, 12, 24, 36, and 48 weeks. At each visit, we measured the ulcer with a specialized camera, and associated software and the area (cm2) was calculated. Patients and assessors were blinded to treatment allocation. We followed all patients for 48 weeks or until wound healing or surgical treatment. Findings/results. We included 48 patients in the analysis (24 in each group), with a total of 64 ulcers. Among them 41 ulcers were followed until healing or 48-week follow-up and 20 ulcers were surgically treated during the study period. Three patients were lost for follow-up. The intention-to-treat analysis showed a significantly higher rate of ulcer healing in the high-dose group with 21 of 30 (70%) healed ulcers compared to 12 of 34 (35%) in the low-dose group (P = .012). Median ulcer reduction at final follow-up was 100% (interquartile range [IQR]: 72-100) in the high-dose group and 57% (IQR: −28 to 100) in the low-dose group. Furthermore, we found a significant effect of high-dose vitamin D on ulcer reduction in the repeated measures analysis of variance. Conclusions. We found high-dose vitamin D3 to be efficient, compared to low-dose vitamin D3, in promoting healing in chronic diabetic foot ulcers.