In 14 patients with hypertensive crisis treated with diazoxide, close monitoring of blood pressure, heart rate, and symptoms was performed. Standard 12-lead electrocardiograms were recorded before and after diazoxide. All patients showed a significant fall in blood pressure after drug administration. Seven patients (50%) showed significant ST-T changes after diazoxide. Six patients (43%) developed substernal discomfort demonstrated by substernal pain of tightness. Five patients (35%) had both chest discomfort and ST-T changes. One of these patients with substernal pain and ST elevation had evidence of acute myocardial infarction with serial enzyme studies. In the patients with significant ST-T changes, the average fall in blood pressure was significantly greater than the average fall in blood pressure in the patients without significant ST-T changes. These findings suggest that both ST-T changes and substernal discomfort were due to myocardial ischemia secondary to a sudden severe drop in blood pressure.