Study aim: To evaluate the effect of vitamin D supplementation on the severity, duration and frequency of headache attacks in adolescents with migraine. Design: A randomized; parallel group; double-blind (outcome assessor and statistical analyst blinded); phase 3 clinical trial of 112 patients. Randomization was performed using a permuted block method with a block size of 4 to ensure concealed allocation sequence. Settings and conduct: Setting: Pediatric Neurology Clinic of ValiAsr Hospital of Birjand; one hundred and twelve adolescents randomized into two groups; outcome assessor and statistical analyst are blinded. Participants/Inclusion and exclusion criteria: Inclusion criteria: Age twelve to eighteen years; migraine diagnosis; stable Sodium Valproate dose. Exclusion criteria: Renal disease; recent supplement use; high serum calcium. Intervention groups: Intervention group: Receiving routine treatment with Sodium Valproate tablets; plus weekly consumption of one Vitamin D softgel containing 50000 International Units for 10 weeks. Control group: Receiving routine treatment with Sodium Valproate tablets only; without receiving any additional supplements or placebos. Main outcome variables: Mean headache intensity score; time of measurement at baseline and 10 weeks after the initiation of the intervention; measurement tool is the Visual Analog Scale from score 0 to 10.