We studied the effect of adding positive end expiratory pressure, PEEP, during the administration of a nebulized bronchodilator in eight patients with acute respiratory failure and severe bronchospasm. Respiratory failure was diagnosed by means of an arterial blood gas analysis and bronchospasm by the presence of wheezing, dyspnea, cough and a forced expiratory volume in the first second, FEV1, of less than 0.7 L. A randomized cross over design was used, where each patient was subjected to two PEEP treatments and two control treatments with zero end expiratory pressure, ZEEP. The interval between each treatment was three hours. With ZEEP, FEV1 rose from 0.69 +/- 0.31 to 1.03 +/- 0.23 L and with PEEP from 0.66 +/- 0.28 to 1.50 +/- 0.48 L (P less than 0.05). Similar changes were seen in forced vital capacity and in peak flow. These results demonstrate that PEEP improves the efficacy of an inhaled, nebulized bronchodilator. This effect is probably mediated through a better distribution especially in peripheral airways.