Intravenous magnesium sulphate (MgSO4) has been tried in the emergency department treatment of asthma since the mid-1980s, but published reports vary as to its efficacy. The literature suggests that it may be effective in the more severely ill asthmatic. We evaluated i.v. MgSO4 in adult asthmatics having a moderate to severe exacerbation. The study was performed in a convenience sample of adult asthmatics between the ages of 18 and 55 presenting to the emergency department with a peak expiratory flow (PEF) of < 100 l/min or < 25% of predicted flow. Patients received either 2.0 grams of MgSO4 or placebo in a randomized, double-blind fashion. All patients received inhaled bronchodilators and i.v. steroids. Outcome variables were: improvement in PEF, subjective respiratory distress as measured by the Borg dyspnoea scale (BDS) and hospital admission. The first visits of 42 patients presenting with acute asthma exacerbations were evaluated, 18 receiving MgSO4 and 24 receiving placebo. The t = 60 peak flow in the MgSO4 group was 174 l/min versus 212 l/min in placebo, p = 0.04. Controlling for age, heart rate, initial PEF and initial BDS in ordinal logistic regression, the t = 60 Borg scale of subjective dyspnoea had an odds ratio of 1.54 in favour of more dyspnoea in MgSO4 (95% C.I., 0.36-6.67; p = 0.56). Five of 18 patients (28%) receiving MgSO4 were admitted compared with 5 of 24 (21%) receiving placebo (p = 0.72). In moderately severe adult asthmatics, 2.0 grams of MgSO4 i.v. resulted in less improvement in peak expiratory flow compared with placebo. MgSO4 did not appear to decrease subjective dyspnoea or the hospital admission rate. This evidence does not support the use of MgSO4 in the treatment of acute asthma.