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 β-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. Interventions. 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.