Fifty-six patients with a preliminary diagnosis of possible acute myocardial infarction (AMI) were studied on the second or third day after onset of symptoms by 99mtechnetium stannous pyrophosphate myocardial imaging. The scintigraphy was positive in 25 (44.6%). The final clinical diagnoses upon discharge were: definite AMI in 11 with positive scintigraphy in 9 (82%), intermediate coronary syndrome (ICS) in 37 with positive scintigraphy in 15 (40.5%), postinfarction failure in 4 with positive scintigraphy in 1, no diagnosis of coronary heart disease in 4 patients with negative scintigraphy in all. Of the 37 patients with a final diagnosis of ICS, 25 were admitted to the Coronary Care Unit with chest pain as the only symptom. In this group the mean percentage increase in ASAT was significantly higher in 9 patients with positive scintigrams than in 16 with negative. It is therefore assumed that among patients with ICS, a positive scintigraphy may reflect a more serious myocardial injury than a negative scintigram. Of six patients with an acute tachyarrhythmia and ICS, scintigraphy was positive in the three with the most long-lasting or severe arrhythmmias. False negative scintigrams may be seen in some patients with definite AMI.