The ability of patients with severely impaired left ventricle function to perform short-term exercise and to participate in a cardiac rehabilitation program and attain physical training effects was evaluated. Treadmill exercise tests were performed before and after physical conditioning in 10 patients with a prior myocardial infarction and a left ventricular ejection fraction at rest of less than 27 percent (range 13 to 26) determined by radionuclide angiography. All patients participated in a supervised exercise program with a follow-up period of 4 to 37 (mean 12.7) months. Baseline exercise capacity showed marked variability, ranging from 4.5 to 9.4 (mean 7.0 +/- 1.9) METS, and improved to 5.5 to 14 (mean 8.5 +/- 2.9) METS after conditioning (p = 0.05). The oxygen pulse (maximal oxygen uptake/maximal heart rate) before and after conditioning was used to assess a training effect and increased significantly from 12.8 +/- 2.0 to 15.7 +/- 3.2 ml/beta (p less than 0.01). There was no exercise-related morbidity or mortality, although two patients died during the study period. It is concluded that selected patients with severely imparied left ventricular function can safely participate in a conditioning program and achieve cardiovascular training effects.