The use of diazoxide in the treatment of pulmonary hypertension has been advocated recently. We describe three patients who responded less favourably. The first patient had cardiac arrest (asystole) after the acute bolus dose. After recovery only a slight increase in cardiac index was seen while pulmonary vascular resistance was unchanged. The second patient developed total atrioventricular block after the acute bolus dose. Ater recovery the cardiac index was unchanged while pulmonary vascular resistance was decreased. The third patient felt ill after the acute bolus dose, and the blood pressure dropped, but no conduction abnormalities were noted. The cardiac index rose slightly and the pulmonary vascular resistance was essentially unchanged. Long term treatment with oral diazoxide could not be administered to this patient because abdominal pain and distension developed. Because we lack reliable alternative treatment, it is justifiable to try diazoxide, provided great care is taken.