Three patients with primary pulmonary hypertension and one man with suspected thromboembolic pulmonary hypertension were given up to 300 mg diazoxide by injection into the pulmonary artery. The three patients with primary pulmonary hypertension responded with a fall in total pulmonary resistance and a rise in cardiac output. The patient with suspected thromboembolic pulmonary hypertension did not respond. Haemodynamic and clinical improvement was maintained by oral diazoxide in two of the responders, and in one the prognosis appeared to be greatly improved.