A 6-month double-blind study was conducted in 26 patients suffering from a severe obstructive disturbance of ventilation and respiratory insufficiency to find out the behaviour of blood gases and pulmonary haemodynamics under Almitrin treatment compared with placebo. The acute administration of 75 mg Almitrin resulted in a significant increase of PaO2 by 11.7 mmHg, as well as of PAPR by 4.4 mmHg. Statistical significance was seen only for the systolic pressure of the a. pulmonalis. After subsequent 6-month treatment with 3 x 25 mg Almitrin, a significantly higher PaO2 is seen at T6 than in the control group. Another additional administration of 75 mg Almitrin results in a further increase of PaO2 by 7.7 mmHg. At T6 the behaviour of pulmonary haemodynamics is identical in both groups A and P.