The effects of different doses of nifedipine on frequency of angina and objective measurements of myocardial ischaemia during exercise were studied in 10 patients with stable angina pectoris. In a single blind trial over nine weeks patients received one week's treatment each with placebo, nifedipine 10 mg, 20 mg, 30 mg, 40 mg, 30 mg, 20 mg, 10 mg, then placebo three times a day. The response to the different doses of nifedipine was highly variable. On exercising, three patients achieved a consistent improvement in workload attained before onset of ST segment depression and maximum ST depression during exercise testing during all active phases. Four patients improved with 10 mg three times a day but deteriorated at higher doses. In two patients there was no objective or subjective improvement with any dose of the drug, while in one patient anginal frequency increased and there was objective deterioration during exercise testing at doses above 10 mg three times a day. Thus a dose of nifedipine that is beneficial in one patient may have minimal or opposite effects in another. These results indicate the importance of careful titration of doses for individual patients if the maximum benefit from nifedipine is to be obtained in patients with exertional angina.