1. This placebo-controlled study assessed once daily sustained-release (SR) diltiazem, 200 and 300 mg, in 182 stable angina patients with positive exercise test. 2. Exercise testing was performed at baseline after a 7 day placebo run-in period, and repeated after 7 days of treatment, 25.0 +/- 0.1 h postdose. 3. Diltiazem (200 and 300 mg) produced respectively a 68% and a 64% decrease in weekly angina episodes, and placebo a 15% decrease (P < 0.05). Similarly, both dose levels produced a 70% decrease in nitroglycerin consumption, whereas no difference was obtained with placebo (P < 0.01). The increase in time to ischaemic threshold was significantly superior for 200 mg and 300 mg diltiazem when compared with placebo (75.2 and 91.5 s respectively vs 47.0 s) (P < 0.05); increase in time to anginal threshold was also significantly greater for diltiazem when compared with placebo (84.6 and 85.9 s respectively vs 43.9 s) (P < 0.05). 4. Only one patient experienced worsening of angina and had to be withdrawn from the study. 5. This study demonstrates 200 and 300 mg SR diltiazem is effective when given once-a-day in the prophylaxis of stable exertional angina. This once daily formulation should improve patients' compliance and comfort.