Thallium-201 scintigraphy in combination with intravenous dipyridamole has been reported to be useful in patients who are unable to perform maximal exercise stress testing. Few reports have dealt with side effects in large numbers of patients. For that reason noncardiac side effects, cardiac effects, electrocardiographic changes, as well as hemodynamic changes were studied in 301 consecutive patients, examined by dipyridamole thallium-201 imaging because of suspected coronary artery disease. The patients were divided into two groups (A and B). Patients in group A (101 patients) received an infusion of 0.14 mg/kg per minute dipyridamole for four minutes. Patients in group B (200 patients) received the same dose of dipyridamole followed by low level exercise (60 rpm/30 Watts) for three minutes to achieve maximal coronary blood flow and to diminish thallium-201 uptake in the gastrointestinal organs. The likelihood of having hemodynamically significant coronary artery disease was the same in both groups. Patients in group B experienced significantly less side effects than patients in group A (11% vs 43%, P less than 0.05). In group B changes in systolic blood pressure (P less than 0.05), heart rate (P less than 0.05), and rate pressure product (P less than 0.05) were more distinct. The occurrence of angina was the same in both groups (47% vs 44%, NS), but ischemic ST segment changes were significantly more frequent in group B than in group A (25% vs 12%, P less than 0.05). Thus, exercise added to dipyridamole infusion compared to dipyridamole infusion alone results in fewer noncardiac side effects, the same prevalence of angina pectoris, and a higher incidence of ST segment changes.