BACKGROUND: Patients with syndrome X (exertional angina, positive exercise test, normal coronary arteriogram) have an altered perception of cardiac pain. This symptom may arise from increased sensitivity to adenosine. Previous studies suggest that intravenous aminophylline (an adenosine receptor blocker) improves exercise tolerance in patients with this disorder. OBJECTIVE: To examine the efficacy of oral aminophylline in syndrome X. METHODS: 13 patients (11 women and two men, mean (SD) 54 (6) years) with syndrome X were studied. Patients were randomised in a double blind crossover study to receive either oral aminophylline or placebo for three weeks. All patients underwent symptom limited exercise testing and ambulatory electrocardiography at the end of each three week period. RESULTS: 10 patients completed the study. The time to angina during exercise testing in patients who were given aminophylline was longer than for the placebo group (mean (SD) 632 (202) seconds v 522 (264) seconds, P = 0.004). Peak exercise ST depression did not differ significantly between patients who received aminophylline and those administered placebo (mean (SD) -1.9 (0.7) mm v -1.5 (0.8) mm). Six patients taking aminophylline reported a reduction in the total number of episodes of chest pain during the three weeks, but the frequency and duration of ST segment depression during Holter monitoring was unchanged. CONCLUSION: Oral aminophylline has a favourable effect on exercise induced chest pain threshold in patients with syndrome X. The disparate effects on symptoms and ST segment changes are intriguing and further study is warranted.