In a group of 23 patients with first-time myocardial infarction (MI) we compared the results of echocardiography and chest X-ray as measured 1 week, 2 months, and 6 months following acute MI. Left ventricular end-diastolic dimension (LVEDd) and left atrial (LA) dimension were measured from the echocardiogram, and the cardiac volume in ml/m2 body surface area (BSA) was calculated from the chest X-ray. A progressive increase in LA dimension was noticed during the 6-month period: a significant increase after 2 months (P less than 0.001) with a further increase at 6 months compared with after 2 months (P less than 0.001). The changes in LA dimension were more pronounced in anterior and Q-wave infarction (P less than 0.001) than in inferior and non-Q-wave infarction (P less than 0.01). On the other hand, LVEDd showed a less conspicuous change: a moderate increase (P less than 0.05) at 2 and 6 months, also with a more pronounced change in anterior wall and Q-wave infarction (P less than 0.01). There was no significant concurrent change in the calculated heart volume in ml/m2 BSA, as measured from the chest X-ray. It is suggested that the observed changes in LA dimension reflect reduced left ventricular compliance after MI.