Aethophylline and aminophylline aerosols were investigated using a jet or an ultrasonic nebulizer in 51 patients suffering from NSLD with reversible bronchospasm. FEV1 [forced expiratory volume in 1s] was controlled in fixed intervals, and the patients made subjective statements. A bronchospasmolytic test with isoprenaline spray at the end demonstrated the actual reversibility of the bronchospasm. Only theophylline aerosols produced by an ultrasonic nebulizer had a subjective and a functional efficacy. Theophylline aerosols of a 2.5% solution were better than aerosols of a 5% solution, but were less effective in comparison with the sympathicomimetic drug isoprenaline spray. There was a good correlation with the subjective statements of the patients concerning the clinical effects. The concentration of the therapeutic aerosol was an important factor for the local tolerance and the ventilatory effects; measurable serum theophylline levels could not be found. The topical action of theophylline is responsible for the clinical effect.