The efficacy and safety of bepridil hydrochloride (200 to 400 mg/day) were evaluated in patients with chronic stable angina refractory to maximal tolerated doses of diltiazem (median 360 mg/day) in a randomized, multicenter, double-blind, parallel study. Baseline diltiazem data were obtained during a 2-week period, after which 86 patients were randomized to bepridil (n = 46) or diltiazem (n = 40). Angina frequency, nitroglycerin consumption and ischemic manifestations induced by exercise treadmill testing were evaluated over 8 weeks. Bepridil significantly (p less than 0.05) increased time to angina onset, time to 1 and 2 mm of ST-segment depression, total exercise time and total work over baseline values. Changes in time to angina onset and time to 1 mm of ST-segment depression were significantly (p less than 0.05) greater for bepridil than for diltiazem. Angina frequency and nitroglycerin consumption did not differ significantly between groups. Compared with baseline, bepridil significantly (p less than 0.001) decreased heart rate (mean 4 beats/min) and prolonged QTc (mean 35 ms). The most frequent adverse effects in both groups were nausea, asthenia, dizziness, headache and diarrhea. Four patients taking bepridil and 1 taking diltiazem withdrew from the study because of adverse reactions. No sudden deaths, myocardial infarctions or instances of sustained ventricular tachycardia or torsades de pointes occurred in either group. The data indicate that bepridil provided safe and effective antianginal and antiischemic therapy in patients with chronic stable angina who exhibited less than optimal response to maximal tolerated doses of diltiazem.