The clinical efficacy and safety of continuous inhalation therapy using l-isoproterenol in adult acute severe asthma were retrospectively evaluated. Fifty adult patients (34 men and 16 women, mean age 47.5) with severe acute asthma, which was not improved by conventional therapy including intermittent inhalation of salbutamol, intravenous aminophylline, and corticosteroid, were given continuous l-isoproterenol inhalation treatment. Both the lowered PaO₂/FiO₂ and the elevated PaCO₂ improved significantly within 4 hours later. Forty-seven patients (94.0%) showed clinical improvement whereas two (4.0%) did not respond and received mechanical ventilation. No obvious side effects were noted in 49 patients (98.0%), while an 81-year-old man showed an elevation of blood pressure, which recovered promptly after discontinuation of the treatment. Based on these observations, we conclude that continuous inhalation therapy using l-isoproterenol is a safe and possibly useful option in the treatment of severe adult acute asthma, especially in patients who do not respond to conventional therapy.