Left ventricular (LV) function is an important predictor of morbidity and mortality after myocardial infarction (MI). Changes in LV function have been examined during the early and late phases after MI, but serial measurements of LV function during the subacute period have not been performed. To assess sequential changes in LV function during the subacute period after MI, we used quantitative two-dimensional echocardiography to examine 22 patients over a 1-year period. Twenty-one of the 22 patients had a Q-wave MI. Eleven had an anterior MI and 10 had an inferior MI; their peak creatine phosphokinase (CPK) was 1213 mIU/ml +/- 14. Three weeks after acute MI, LV ejection fraction (LVEF) had increased from 45% to 52%. Seven of 19 patients showed an LVEF < 43% at baseline. In five of these patients, LVEF improved, but in two patients, LVEF was still < 43% in week 3. There was a significant enlargement of LV end-diastolic volume (LVEDV) (94 ml to 112 ml, p < 0.05) across the four observations but no change in LV end-systolic volume (LVESV; 54 ml to 56 ml, p = n.s.). When two groups (G1 [depressed], LVEF < or = 43%; G2 [preserved], LVEF > 43%) were compared, the group with depressed LVEF demonstrated a higher probability of improvement in LVEF (34% to 47%, p < 0.001) and stroke volume (38 ml to 65 ml, p < 0.01).