This study was designed to investigate the effects of acute left atrial hypertension on left atrial size. Twenty-four patients with acute myocardial infarction were studied. The estimated mean left atrial pressure (LAm) was correlated with left atrial (LA) size obtained by echocardiogrphy. The LAm was elevated (greater than 12 mmHg) in 15 patients (Group I). The LA size was within normal limits in all but two patients who had minor increases. The LAm was normal in nine patients (Group II). The LA size was normal in each case. The LA size remained unchanged in those patients who had a stable LAm. We conclude that acute increases in LAm are not usually associated with LA enlargement beyond the upper limit of the normal range.