Dilation of the left ventricle after myocardial infarction is common, occurs rapidly (within 2 weeks of infarction) and may be self-limited. To evaluate the time course of postinfarction left ventricular dilation and to assess the impact of successful coronary thrombolysis, serial radionuclide left ventricular volume analyses were performed in 36 patients undergoing attempted thrombolysis for acute transmural myocardial infarction. All patients underwent cardiac catheterization, coronary angiography and attempted thrombolysis within 7 h of the onset of symptoms. The site of coronary occlusion was the left anterior descending coronary artery in 17 patients, the right coronary artery in 18 and, in 1 patient, occluded bypass grafts to the right and left circumflex coronary arteries. Attempted reperfusion using a thrombolytic agent was successful in 22 individuals, occurring 5 ± 1 h after the onset of symptoms. Gated radionuclide ventriculography was performed early (mean time 1 day after admission, n = 36), subacutely (mean time 11 days postinfarction, n = 36) and late after infarction (mean time 10.5 months, n = 25), and a geometric technique was used to measure serial left ventricular enddiastolic volume. Left ventricular end-diastolic volume for the entire group increased significantly (p < 0.01) from 153 ± 30 ml at baseline to 172 ± 45 ml (at 11 days) to 220 ± 63 ml (at 10.5 months). Twenty of 36 patients showed >20% increase in left ventricular end-diastolic volume (dilation) with time. This appeared early in seven patients, occurred remote from infarction in seven others and showed a progressive pattern in six. Left ventricular dilation was more frequent and chronic dilation significantly more marked (p < 0.01) in patients with left anterior descending as compared with right coronary artery occlusion. Dilation outcomes and ejection fraction for patients in whom attempted thrombolysis was not successful were similar to findings in patients for whom reperfusion was successful. It is concluded that left ventricular dilation after myocardial infarction is a common, though sometimes late-appearing complication, both more frequent and severe in anterior infarction. Relatively late thrombolysis (5 ± 1 h) in the course of acute myocardial infarction did not prevent subsequent development of left ventricular dilation, most probably because of delayed intervention.