Thallium-201 myocardial imaging was performed in seven long-term cardiac transplant survivors who were undergoing selective coronary arteriography as a screening test for immune mediated coronary atherosclerosis. The radionuclide studies showed fixed defects in all patients, though none was known to have had a myocardial infarction. Such abnormalities may be due to previous rejection episodes. Reversible ('ischaemic') defects were present in two patients with triple vessel disease, and were also seen in three subjects with minor (less than 50% luminal diameter) stenosis. Ischaemic thallium-201 defects in cardiac transplant recipients indicate the presence of coronary artery disease, though the stenosis may be of a degree which would normally be considered haemodynamically non-significant. The possible role of small vessel disease in producing these defects is discussed.