The vasodilatory effect of magnesium sulfate (MgSO4) in cerebral vessels has been previously demonstrated. Our prospective, randomized study assessed the effect of MgSO4 in the treatment of vasospasm in patients with spontaneous subarachnoid haemorrhage (SAH). Seventy-four patients with SAH were randomly divided into 3 groups. In Group A, only nimodipine was administered; in Group B, only MgSO4 was given; and in Group C, both nimodipine and MgSO4 were administered. Daily TransCranial Doppler (TCD) measurements of the anterior (ACA) and middle (MCA) cerebral arteries were subsequently obtained. Glasgow Outcome Scale (GOS) scores, hospital stay length, and the cost of treatment were tracked and calculated. Mean flow velocity measurements for ACA and MCA were calculated. Differences between Groups A and B, and Groups A and C (p= 0.0013, 0.0011, respectively) were statistically significant. The mean GOS scores were: Group A, 3.8; Group B, 4.4; and Group C, 4.1. The mean lengths of stay were: Group A, 11.8 ± 0.2 days; Group B, 11.5 ± 0.2 d; and Group C, 11.3 ± 0.1 d. The cost of treatment was similar between all groups. Intravenous MgSO4 significantly decreases cerebral flow velocities. Administration of MgSO 4 improved our patients' outcomes and reduced the length of their hospital stay. Our preliminary results justify the need for a large, randomized multi-institutional study.