Aminophylline was known to decrease effort angina and ischaemia in patients with coronary artery disease and stable angina. This effect has been explained by the antagonization of the vasodilatation caused by adenosine and the prevention of the transmural myocardial maldistribution (steal phenomenon). In this study, treadmill exercise tests (with Bruce protocol) were performed in 14 patients (11 women, 3 men, mean age 51 +/- 9.9 years) diagnosed with syndrome X (typical anginal chest pain, abnormal stress test, normal coronary angiography) before (basal condition) and after the acute i.v. aminophylline infusion (total dosage 6 mg/kg within 15 minutes). It was shown that aminophylline lengthened the time before the occurrence of ischaemia in patients with syndrome X by increasing the ischaemia threshold in spite of the occurrence of tachycardia but it had no effect on the total exercise duration. Aminophylline infusion also exerted a beneficial effect on exercise-induced chest pain. It has been suggested that the role of the adenosine receptor could be important in the pathogenesis of syndrome X.