Ten patients four men and six women aged 35 to 78 years (mean, 57) who had been admitted to a coronary care unit because of unstable angina, received intravenous infusions of diltiazem for three to ten days after a 24-hour observation period. During the observation and treatment periods, the patients were monitored daily for number of attacks of angina, nitroglycerin intake, blood pressure, and ECG parameters. Routine laboratory tests were performed before and after diltiazem therapy. Two patients did not respond to treatment, but administration of diltiazem controlled anginal pain in eight patients, without producing side effects. In one case the dose had to be reduced because of atrioventricular dissociation; in three cases there was a modest increase in SGPT. Diltiazem was found to be a safe and effective drug for controlling anginal attacks in patients with unstable angina.