To assess the possible role of variables not related to early infarct artery reperfusion in predicting late changes in ventricular function after infarction, paired early (mean 6.6 +/- 3.5 days after admission) and late (12.7 +/- 7.0 months later) cross-sectional echocardiograms from 54 infarction survivors were retrospectively reviewed. Ejection fraction was calculated from digitized biapical echocardiographic views on a graphics tablet. Changes of 0.10 or more in LVEF were correlated with 23 clinical variables. By stepwise regression analysis, Q-wave infarction and low early LVEF independently predicted late improvement in function. Early high LVEF and interval infarction were the only independent predictors of late declines in function. Overall, when patients were indexed by early left ventricular systolic function, a pronounced late "regression to the mean" was noted with initially high values tending to fall and low values to rise (r = -0.44, p less than 0.001). This effect must be accounted for in any acute intervention trial in myocardial infarction. The occurrence of Q-wave infarction does not exclude late improvement in ventricular function.