STUDY OBJECTIVE: To determine the safety and efficacy of diltiazem CD 180 mg administered once/day in patients with chronic stable angina inadequately controlled with P-blockers, long-acting nitrates, or both. DESIGN: Multicenter, randomized, double-blind, placebo-controlled, parallel-group trial. SETTING: Medical clinics in the private and academic sectors. PATIENTS: Of 172 patients, 170 completed the 2-week double-blind treatment period. INTERVENTION: . Patients received either diltiazem CD 180 mg or placebo once/day in combination with existing antianginal therapy. MEASUREMENTS AND MAIN RESULTS: The time to termination of exercise tolerance testing, 24 hours after the dose increased significantly in the diltiazem CD group (37.2 sec) compared with the placebo group (21.3 sec, p=0.0438). Time to onset of angina during exercise testing also increased (57.6 vs 35.0 sec, respectively, p=0.0324), as did time to moderate angina (37.5 vs 20.6 sec, respectively, p=0.0354). The rates of total angina attacks and of angina attacks on exertion were significantly reduced in the diltiazem CD group versus placebo (p<0.05). Significant reductions in systolic and diastolic blood pressures and heart rate-blood pressure product measured at rest, submaximum exercise, and exercise termination were observed in diltiazem CD-treated patients compared with placebo (p<0.05). The frequency of treatment-related adverse events was identical in the two groups, 15.1%. CONCLUSION: Diltiazem CD 180 mg once/day is an effective, safe, and beneficial initial dosage when added to existing antianginal therapy.