In a clinical study, the value of thallium-201 reinjection was studied in 139 patients with suspected or known coronary artery disease who showed one or more persistent defects after conventional stress-redistribution imaging. Fifty-nine (42%) patients had sustained a Qwave myocardial infarction. Sixty-eight (49%) patients showed a reversible defect in at least one myocardial segment at redistribution, while 71 (51%) had persistent defects only. Following reinjection additional segmental filling-in was seen in 95 (68%) patients, including 50 of the 68 (74%) patients with reversible defects and 45 of the 71 (63 %) with persistent defects only. On the immediately post-exercise images, 458 (47%) of 973 segments showed perfusion defects. At redistribution 105 (23%) of the 458 defects showed filling-in, whereas of the remaining 353 persistent defects 164 (46%) resolved additionally after reinjection. Thirteen (10%) of 133 Q-wave related defects showed filling-in at redistribution compared with 22 (27%) of 82 remote defects (P =0.001). After reinjection additional filling-in of defects was seen in 47 (39%) of 120 Q-wave related defects compared with 35 (58%) or 60 remote defects (P =0.015). Overall, 60 (45%) of 133 Qwave related defects resolved compared with 57 (70%) of 82 remote defects (P = 0.005). Thus reinjection of thallium-201 (1) revealed reversible defects in 63% of patients with only persistent defects at redistribution, (2) demonstrated additional filling-in in 74% of patients who already showed reversible defects at redistribution, (3) showed viability in 46% of segments initially classified as persistent, and (4) demonstrated filling-in in 39% of Qwave related segments, although this was less conspicuous than in remote segments.