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In order to compare the efficacy of nifedipine gastrointestinal therapeutic system (GITS) with diltiazem, 99 patients with chronic stable angina were studied in a parallel-group randomised trial. According to the results of the two exercise tolerance tests (ETTs) performed during the placebo run-in, patients were divided into a fixed threshold group if the variability in time to 1 mm ST-segment depression was 20%, or a variable threshold group if it was higher. Efficacy was assessed by comparing the time to 1 mm ST-depression on a bicycle ETT after 4 weeks of treatment, adjusting for the baseline value. The adjusted means were 7.44 min for nifedipine GITS and 7.68 min for diltiazem; the difference was -0.24 min, with a lower 90% confidence limit of -0.90, which is within the stated interval for equivalence. The same result was confirmed by the 'intention-to-treat' analysis, and comparable results were obtained both in fixed and in variable threshold groups. The incidence of side effects was 12% with nifedipine GITS and 8.2% with diltiazem. Nifedipine GITS and diltiazem were found to be equally effective in increasing exercise tolerance in chronic stable angina patients.