To identify patients at risk of cardiac expansion during hospital stay for a first acute myocardial infarction (AMI), 41 patients underwent right-sided cardiac catheterization soon after admission and serial 2-dimensional echocardiography on days 1, 3 or 4 and between days 7 and 10. Infarct expansion was recognized by echocardiography in 11 patients (27%), most often on the second recording (day 3 or 4). Age, sex, time from onset of pain to catheterization, peak levels of creatine kinase and creatine kinase-MB isoenzyme, heart rate, mean pulmonary artery wedge pressure and left ventricular stroke work index were similar in the 2 groups. Patients in whom infarct expansion developed had a higher incidence of previous systemic hypertension (73% vs 27%, p less than 0.01) and anterior AMI (91% vs 30%, p less than 0.001) and a higher mortality rate at 1 year (73 vs 7%, p less than 0.001) than those who did not. They also had higher systolic (139 +/- 24 vs 126 +/- 18 mm Hg, p less than 0.05) and diastolic (91 +/- 14 vs 75 +/- 13 mm Hg, p less than 0.001) arterial pressures, lower stroke volume index (31 +/- 10 vs 40 +/- 10 ml/m2, p less than 0.01) and much higher systemic vascular resistance (SVR) values (1,713 +/- 380 vs 1,253 +/- 264 dynes s cm-5, p less than 0.0001). In the subgroups of patients with anterior AMI, differences were significant for diastolic arterial pressure, stroke volume index, SVR and mortality.(ABSTRACT TRUNCATED AT 250 WORDS)