Magnesium sulphate has been used in the acute treatment of migraines; some studies found it to be a highly effective medication in the acute control of migraine pain and associated symptoms. This randomized, double-blind, placebo-controlled study assesses the effect of magnesium sulphate on the pain and associated symptoms in patients with migraine without aura and migraine with aura. Sixty patients in each group were assigned at random to receive magnesium sulphate, 1000 mg intravenously, or 0.9% physiological saline, 10 ml. We used seven parameters of analgesic evaluation and an analogue scale to assess nausea, photophobia and phonophobia. In the migraine without aura group there was no statistically significant difference in the patients who received magnesium sulphate vs. placebo in pain relief. The analgesic therapeutic gain was 17% and number needed to treat was 5.98 at 1 h. There was also no statistical difference in relief of nausea. We did observe a significant lower intensity of photophobia and phonophobia in patients who received magnesium sulphate. In the migraine with aura group patients receiving magnesium sulphate presented a statistically significant improvement of pain and of all associated symptoms compared with controls. The analgesic therapeutic gain was 36.7% at 1 h. A smaller number of patients continued to have aura in the magnesium sulphate group compared with placebo 1 h after the administration of medication. Our data support the idea that magnesium sulphate can be used for the treatment of all symptoms in migraine with aura, or as an adjuvant therapy for associated symptoms in patients with migraine without aura.