To determine the value of thallium-201 scintigraphy as a decisive factor in admission policy for patients with acute chest pain and nondiagnostic electrocardiograms, we undertook a prospective study in 149 such patients. The interval between pain and scan never exceeded 12 hr. Of 57 patients in whom a defect was seen, 34 had an acute infarction, 7 developed infarction within 2 months, and in 11 coronary heart disease was proven by angiography or strongly suggested by stress tests (ECG and thallium-201 scan). In 13 patients with an equivocal scan, coronary heart disease was proven or strongly suggested in 5. Of 79 patients with a normal scan, only 1 had acute infarction, and stress tests were positive in 6 and negative in 72. In these 72 no cardiac event occurred during a 1-year follow-up. Thallium-201 scintigraphy can help to select those patients with acute chest pain and nondiagnostic electrocardiograms who need observation in a CCU.