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To determine the natural history of left ventricular function at rest and during exercise and to assess the impact of this variable on subsequent mortality, 165 patients were studied with radionuclide angiography within 24 hours of acute myocardial infarction. The ejection fraction of the 19 patients who died during the 6 month follow-up was lower than that of the 146 survivals: 41 +/- 16% vs 50 +/- 13% (P less than 0.001). Before hospital discharge (14 +/- 4 days), 83 patients had a rest and submaximal exercise radionuclide study. The ejection fraction of the 42 patients with anterior infarction was 44 +/- 12% and remained unchanged during exercise, while the 41 patients with posterior infarction had a resting value of 54 +/- 9% which increased to 57 +/- 10% (P less than 0.001) during exercise. The ejection fraction during exercise increased slightly but significantly in 37/61 patients with single vessel disease, while it did not change in the 24/61 patients with multivessel disease. At a mean of 4 +/- 1 months following infarction, 58 patients underwent a symptom-limited exercise radionuclide study. Mean value of resting ejection fraction for the group or anterior-posterior infarction subgroups did not change from initial or predischarge values. The 27 patients with anterior infarction showed no change in ejection fraction during exercise, while the 31 patients with posterior infarction increased their ejection fraction from 53 +/- 11% to 57 +/- 12% (P less than 0.001). Thus, ejection fraction measured by radionuclide angiography 24 hours following acute myocardial infarction provides useful prognostic information.(ABSTRACT TRUNCATED AT 250 WORDS)