Aminorex (2-amino-5-phenyl-2-oxazoline) is an appetite-suppressing drug which was available in Switzerland from November 1965 to October 1968. In 1967 a sudden 20-fold increase in the incidence of primary pulmonary hypertension was observed in a Swiss medical clinic. It was noticed that a considerable number of these patients had taken aminorex to reduce weight. A similar increase in the incidence of primary pulmonary hypertension was encountered in other clinics in Switzerland, and also in Austria and Germany, where aminorex was available. An increased incidence of the disease has not been reported in countries where this drug was not available. A decline in the incidence of primary pulmonary hypertension has occurred in Switzerland since the withdrawal of aminorex. We have administered a high oral dose of aminorex to rats for up to 43 weeks and to dogs for 20 weeks. A detailed quantitative pathological examination of the heart and pulmonary vasculature in these animals has failed to reveal any evidence of hypertensive pulmonary vascular disease. Although there is statistical evidence linking aminorex with pulmonary hypertension, there is no proof that aminorex causes hypertensive pulmonary vascular disease in man. It is nevertheless important to enquire into the diet and history of drug ingestion in any patient with unexplained pulmonary hypertension.