In 21 patients with proved acute myocardial infarction, the size of the infarct was estimated with serial magnetic resonance (MR) imaging after intravenous injection of gadolinium diethylenetriaminepentaaceticacid (DTPA) (0.2 mmol per kilogram of body weight). Early reperfusion after thrombolytic therapy or percutaneous transluminal coronary angioplasty performed during the acute phase of infarction was documented with coronary angiography in nine patients (group 1). In 12 patients (group 2), no reperfusion was achieved (n = 5) or no thrombolytic therapy was given (n = 7). All group 2 patients were considered to have no reperfusion. Infarct sizes measured with MR imaging were significantly smaller in group 1 than in group 2 at 8 days ± 4 after infarct onset (8% ± 5% vs 15% ± 4%, respectively; P < .001). Serial MR images showed individual variations in infarct size, findings that may be clinically significant. Estimation of infarct size with Gd-DTPA-enhanced MR imaging is accurate in demonstrating the effect of successful reperfusion therapy on infarct size.