OBJECTIVE--To assess the value of dobutamine over dipyridamole as a pharmacological stressing agent in myocardial perfusion imaging with thallium-201. DESIGN--Stress and redistribution tomographic images were taken in a group of patients in a randomised crossover study of both agents. The scans were scored to give a value for the stress and redistribution images and a reversibility score (redistribution--stress). All patients had coronary angiography that was also scored. Differences between the two agents were compared by a paired t test. PATIENTS--30 patients aged 51-70 years with chest pain thought to be caused by myocardial ischaemia. 11 had had previously myocardial infarction. RESULTS--Dipyridamole caused adverse symptoms in six patients whereas dobutamine caused symptoms in 21 patients (chi 2 = 15.15, p < 0.0001). Dobutamine stress took considerably longer than dipyridamole (31 v 6 minutes) and cost more (17 pounds v 1.50 pounds). There were no significant differences between the agents in terms of total stress or redistribution scores, but regional analysis showed that dipyridamole showed significantly more defects during stress at the apex and lateral wall (p < 0.05), with no significant difference at redistribution. Dipyridamole stress also caused significantly more reversible defects at the apex (p < 0.05) and gave a better correlation than dobutamine with coronary score (dipyridamole r = 0.80, p < 0.001 v dobutamine r = 0.64, p < 0.001). In six patients who had continued to take beta blockers the results of dobutamine stress did not correlate with coronary score, r = 0.34 (NS), whereas dipyridamole studies were not affected. CONCLUSION--Compared with dobutamine, dipyridamole was as effective in producing overall perfusion defects and more effective in provoking defects at the apex and lateral segment. The dipyridamole study correlated better with coronary score and was not affected by concurrent beta blocker treatment. It was also better tolerated by the patients, was less time consuming, and was much cheaper.