Persistent myocardial thallium perfusion defects 3 to 4 hours after stress do not represent irreversible ischemia in every case. In many instances signs of reversibility show up 8-72 hours later or after rest reinjection. Authors performed additional investigations of redistribution in 64 patients with persistent defects. Rest reinjection was given to 11 patients and delayed redistribution was followed in the others. Persistent perfusion defects improved or disappeared in 13 percent of myocardial segments and 38 percent of patients. In 18% of segments late reverse redistribution was observed. The assessment of patients' state was significantly influenced by the results of the additional investigations in 19% of cases. Authors suggest that important information can be obtained about the viability of the myocardium by the extended studies even with the common planar method.