Intravenous magnesium sulphate (MgSO4) has successfully been used in the treatment of acute asthma. The present study investigated the efficacy of nebulized MgSO4 as a bronchodilator in acute asthma as compared to nebulized salbutamol. This was a randomized, double-blind, controlled clinical trial. Asthmatics aged 12-60 yrs in acute exacerbation, with a peak expiratory flow (PEF) <300 L x min(-1), not having taken bronchodilators and not requiring assisted ventilation were included. Patients were randomized to receive treatment with serial nebulizations of either 3 mL (3.2% solution, 95 mg) MgSO4 solution or 3 mL (2.5 mg) salbutamol solution. All patients were also given 100 mg hydrocortisone i.v., and were monitored continuously for 2 h after which they were given supplemental treatment (if and when needed) and either discharged or admitted. Fischl index, PEF improvements (in % predicted) and admission rates were the outcome variables. Thirty-three patients were studied. Fischl score improvement was comparable and significant in both groups (4.31 to 0.43 in the MgSO4 group and 4.29 to 0.76 in the salbutamol group). The increase in PEF was statistically significant and comparable in both groups (by 35% pred in the MgSO4 and by 42% pred in the salbutamol group). Two patients warranted admission in the salbutamol group and one in the MgSO4 group. Nebulized MgSO4 had a significant bronchodilatory effect in acute asthma. This effect was not significantly different from that of nebulized salbutamol.