We studied flow changes, airway pressures, breathing patterns and subjective sensation during tidal breathing on continuous positive airway pressure (CPAP) in 21 acutely ill asthmatic patients and 19 controls. The measurements obtained at various levels of CPAP were compared to the value at zero end-expiratory pressure. The fractional inspiratory time (TI/TTOT) was significantly reduced in both the patients and the control group (p less than 0.01). Patients noticed the best sensation of comfort at CPAP of 5.3 +/- (SD) 2.8 and the control group at 1.6 +/- 2.5. We noted a reduction in peak tidal expiratory flow and an increase in late-phase expiratory flow during tidal breathing in both groups although these changes were not statistically significant. There was improvement in sensation of comfort during low to medium levels of CPAP in acutely ill asthmatics. We conclude that low to medium levels of CPAP may be beneficial in acute asthma by assisting inspiratory muscles. As CPAP is increased, the beneficial effects of increased end-expiratory flow rate may be offset by the reduction in peak tidal expiratory flow rates.