IV magnesium sulfate was administered to a 72-year-old man with acute respiratory failure secondary to a severe asthma attack. The patient had clinically deteriorated despite aggressive standard treatment and evidenced acidosis and hypercarbia by arterial blood gas determination. An IV dose of 1 g MgSO4 produced rapid clinical and arterial blood gas improvement and enabled management of the patient without endotracheal intubation and mechanical ventilation. This is the first reported case of the use of IV MgSO4 to prevent intubation and assisted ventilation in a patient with acute respiratory failure complicating asthma.