We studied the effects of continuous positive airway pressure (CPAP) in 8 asthmatic subjects in whom bronchospasm was induced by aerosolized histamine. The CPAP (12.0 +/- 0.9 cm H2O) increased functional residual capacity by only 0.27 +/- 0.12 L, raised the minimal pleural pressure (Ppl) during inspiration from -32.3 +/- 2.6 cm H2O to -22.8 +/- 2.3 cm H2O (p less than 0.01), and decreased the swings in transdiaphragmatic pressure (Pdi) from 35.1 +/- 2.4 cm H2O to 29.6 +/- 3.7 cm H2O (p less than 0.05). Although ventilation (VE) increased, the inspiratory work per liter of VE fell significantly. More importantly, the pressure-time product for the inspiratory muscles (integral of Ppl.dl) measured over 60 s, fell from 830 +/- 111 to 573 +/- 41 cm H2O.s (p less than 0.05), whereas that for the diaphragm (integral of Pdi.dt) fell from 690 +/- 91 to 497 +/- 74 cm H2O.s (p less than 0.05). We conclude that in induced asthma, CPAP reduces the load on the inspiratory muscles, improving their efficiency and decreasing the energy cost of their action. Our results justify further investigation into the role of CPAP in the treatment of respiratory failure caused by severe bronchial asthma.