The effect of extended-release isosorbide mononitrate (ER-ISMN) on exercise tolerance 1 hour after dosing was compared with that of placebo in a multicenter, randomized, double-blind study of 151 patients with stable effort-induced angina. During a 9- to 24-day placebo run-in, patients underwent Bruce protocol baseline exercise tolerance tests, after which they received ER-ISMN or placebo for 5 days. ER-ISMN patients took 60 mg each morning for the first 4 days and 120 mg on the morning of the fifth day. One hour after dosing, ER-ISMN patients had a significantly greater increase in total exercise time (days 1 to 4:5 ± 53 seconds; day 5: 53 ± 58 seconds) than the placebo-treated patients (days 1 to 4:14 ± 37 seconds; day 5:21 ± 48) (p <0.001). The times to development of angina and 1-mm ST-segment depression were significantly longer in the ER-ISMN group than in the placebo group. The difference between the groups in mean time to onset of angina was 34 seconds after the 60-mg dose (p = 0.004) and 49 seconds after the 120-mg dose (p <0.001). The mean time to development of a 1-mm ST-segment depression was 51 and 61 seconds longer after the 60-mg and 120-mg ER-ISMN doses, respectively, than after placebo (p <0.001). Treatment-related adverse events were reported in 37% (28 of 75) and 7% (5 of 76) of patients in the ER-ISMN and placebo groups, respectively. As expected, headache was more frequent in the ER-ISMN group than in the placebo group (28% and 1%, respectively). The effects of ER-ISMN (60 mg and 120 mg) are clinically evident 1 hour after dosing, resulting in better exercise tolerance in patients with angina pectoris.