OBJECTIVE Intravenous magnesium sulfate (MgSO4), as an adjunctive medication to the standard treatment of acute asthma, improves admission rate or severity score in acute severe asthma patients. METHODS We conducted a randomized double-blind placebo controlled trial with subjects from the emergency room, Ramathibodi Hospital, Bangkok, Thailand. Patients, aged 15-65 years with acute severe asthma attack, whose severity scores were greater than 4 and who were willing to be enrolled in a study during March to November 1997 participated in the study. Randomly allocated patients received either 2 g intravenous MgSO4 or placebo, sterile water, as an adjunctive medication to standard therapy for acute asthma. The medication was diluted in 50 mL of 0.9% normal saline. METHODS Severity scores were measured by two investigators using Fischl's indices. The times interval of measurements were at the initial (0), 60, 120, 180, and 240 min from receipt of treatment. Patients were hospitalized if the severity scores at 240 min exceeded 1. Risk ratio (RR) and 95% confidence interval (CI) of RR were applied to estimate the risk of admission. Analysis of variance with repeated measurement on time was used to determine the severity score between two groups. RESULTS Thirty-four patients with acute severe asthma were enrolled in the present study. One patient from the placebo group was excluded because he did not consent to undergoing peak expiratory flow rate. Seventeen patients received MgSO4 and 16 patients received placebo. The general characteristics between the two groups were not significantly different, which reflected the quality of randomization. The admission rates of the placebo and MgSO4 group were 25.00% and 17.65%, respectively. Patients who received MgSO4 had preventive risk to be hospitalized 0.71 times relative to patients who received placebo. However, this preventive risk did not reach statistical significance (95% CI of RR = 0.19-2.67). The severity score at any time between the two groups was also not statistically significantly different (P = 0.366). CONCLUSIONS With the present evidence, the hypothesis was not confirmed. Magnesium sulphate as an adjunct to standard therapy did not improve either admission rate or severity score in patients with acute severe asthma.