Background The aim of this work was to study changes in end-diastolic volume 6 months after Q-wave and non-Q-wave anterior ST-elevation myocardial infarction by echocardiography. Ventricular dilation after anterior Q-wave myocardial infarction is well-recognized. However, there is a dearth of information about the natural history of ventricular volumes after non-Q-wave myocardial infarction. Methods One hundred ninety patients receiving thrombolytic therapy after anterior ST-elevation myocardial infarction were studied. All patients had 2D echocardiograms and 12-lead electrocardiograms recorded within 24 hours of symptoms and at 3, 42, and 180 days later. In addition, a further electrocardiogram was recorded on day 7 to assess patients for the presence of Q waves. Peak creatine kinase over the first 3 days of admission was recorded. End-diastolic volume index was the study end point. Results Peak creatine kinase was strongly associated with ventricular dilation in both groups (P < .001). Mean end-diastolic volume in the Q-wave group increased significantly from day 1 to 6 months (P < .05) but did not alter after non-Q-wave infarction. However, when patients were selected on predefined criteria for significant change in ventricular dilation (>10 mL/m²), then 35% of those with and 15% of those without Q waves fell into this category. Within this group, the increase in end-diastolic volume followed a similar pattern, with the maximum percentage increase occurring between day 1 and 6 weeks. Conclusions In the postthrombolytic group of anterior ST-elevation myocardial infarction, a minority of patients without Q-wave development also undergo significant ventricular dilation. (Am Heart J 2000;140:611-6.)