We examined the diagnostic usefulness of thallium-201 scintigraphy in 200 patients with acute myocardial infarction. The scintiscans showed a defect, suggesting infarction in 165. In all 44 patients studied iwthin six hours after onset of symptoms the scintiscans indicated a defect. Frequency of positive scans was significantly higher (90 of 96) in patients studied within 24 hours after onset than in those (75 of 104) studied later (p less than 0.01).