In a comparative study, two groups of patients suffering from bronchial asthma or chronic obstructive bronchitis were tested with the new broncholytic substance 7-(3-[2/24/k-dihydroxyphenyl-2-hydroxy-ethylamino]-propyl)-theophylline (reproterol, Bronchspasmin) versus the well proven drug orciprenaline. Both preparations were administered for four weeks in doses of 3 x 20 mg/day (some patients received 6 x 10 mg/day). Different variables of the lung function were examined twice weekly prior to and 1 hr following ingestion of the substance (FVC, FEV1, Rt, IGV, Palpha02). On the basis of findings in above variables, both reproterol and orciprenaline showed very good and significant broncholytic effects. The efficacy of reproterol, however, proved to be significantly superior to that of orciprenaline. This also has been confirmed in subjective reports of patients with regard to strength and duration of alleviation of their respiratory distress. Palpitation, slight tremor of the fingers, restlessness, pressure in the head and dizziness were mentioned as side effects. These symptoms occurred fairly often with orciprenaline, but relatively seldom with reproterol. This difference between the preparations was highly significant. Following four weeks of administration, no tachyphylaxis was observed. In view of the submitted results, reproterol represents, no doubt, a considerable addition to the presently available therapeutic collection of remedies for the treatment of obstructive airways diseases.