An epidemic of chronic pulmonary hypertension of vascular origin (CPHVO) has occurred in Austria, the Federal Republic of Germany, and Switzerland. The epidemic started in 1967 and reached its peak in 1968 and 1969. Since 1972, the prevalence of patients with CPHVO among individuals investigated by cardiac catheterization is again as low as in the pre-epidemic years. In Bern the prevalence of CPHVO during the peak of the epidemic was 20 times higher than during the 12-year period preceding the epidemic. The clinical, physical, electrocardiographic, radiologic, haemodynamic and respiratory findings of the patients observed in Bern (n = 102) are summarized. There has been a mortality between 12 and 20% at the time of the epidemic. Most patients observed for the first time during the epidemic have remained severely disabled over the years. A minute fraction seems to have recovered. There is a close geographic as well as temporal relation of the epidemic to the marketing and intake of the appetite depressing drug aminorex fumarate (Menocil). Acute administration of aminorex leads to a transient rise of the pulmonary artery pressure and vascular resistance in a number of animal species. It has not been possible to produce sustained precapillary pulmonary hypertension and chronic cor pulmonale vasculare under the conditions of chronic administration of the drug in the species tested. Morphologic examination of lung biopsy and autopsy material of patients who have died from CPHVO after the intake of aminorex reveals the presence of "plexogenic pulmonary arteriopathy". The vascular lesions are identical with those observed in pulmonary hypertension due to large congenital left-to-right shunts. In balancing the pros and cons, it appears that the arguments in favour of a cause-effect relationship between aminorex and pulmonary hypertension, which are derived from epidemiological evidence, outweigh the results of "negative" animal experiments. A "propter" in the title of this paper, therefore, seems to be more appropriate than a post".