Twenty-three patients with acute severe asthma were treated in a randomized double-blind way by either intravenous terbutaline two times at an hour interval 6 micrograms/kg or inhaled terbutaline two times 0.1 mg/kg. Peak expiratory flow rate in percent of expected value rose from 89 L/min to 128 L/min (P less than .01) in the intravenous group and from 97 L/min to 122 L/min (P less than .02) in the inhalation group; this was comparable for both groups. PaO2 initially was 7.55 kPa in the intravenous group and rose by 1.4 after 60 minutes (P less than .02) and by 1.33 after 120 minutes (P less than .01). The mean PaO2 did not change in the inhalation group. The pulse rate and blood pressure did not vary significantly in either of the two groups. We conclude that both the intravenous and inhalational administration of terbutaline are effective in acute severe asthma, but that the intravenous route is possibly better for increasing PaO2.