There are inconsistent findings about the efficacy of magnesium in the prophylaxis of migraine attacks and there is no study of magnesium prophylaxis focused on migraine subtypes without aura. In this double blind, randomized, placebo controlled study; we tried to evaluate the prophylactic effects of oral magnesium in migraine patients without aura. The prophylactic effects of 600 mg/day oral magnesium citrate supplementation were assessed by means of clinical evaluation, visual evoked potential and statistical parametric mapping of brain single photon emission computerized tomography before and after a 3 month treatment period. The results of 30 patients with migraine without aura (20-55 years old with 2-5 migraine attacks per month) on magnesium treatment were compared with those of 10 patients with similar properties on placebo treatment. Migraine attack frequency, severity and P1 amplitude in visual evoked potential examination decreased after magnesium treatment with respect to pretreatment values (p < 0.001). In a comparison of the effects of magnesium treatment with those of placebo, post/pretreatment ratios of migraine attack frequency, severity and P1 amplitude in Mg treatment group were found to be significantly lower than those in placebo treatment group (attack frequency p = 0.005, attack severity p < 0.001, P1 amplitude p < 0.05). Cortical blood flow in inferolateral frontal (p < 0.001), inferolateral temporal (p = 0.001) and insular regions (p < 0.01) increased significantly after magnesium treatment with respect to the pretreatment; while such significant changes of cortical blood flow were not observed with placebo treatment. These results have made us think that magnesium is a beneficial agent in prophylaxis of migraine without aura and might work with both vascular and neurogenic mechanisms.