PURPOSE To evaluate the relationship of systolic and diastolic function at rest to exercise capacity. MATERIAL AND METHODS Seventeen patients with ischemic heart failure were included in the study. Ambulatory left ventricular monitoring at rest and during upright exercise with combined analysis of pulmonary gas exchange was performed. Ejection fraction, end-diastolic volume, end-systolic volume, stroke volume, cardiac output, and peak filling rate were measured. RESULTS Significant positive correlations were found between rest ejection fraction and peak oxygen consumption (r = .60, p < .01), peak cardiac output (r = .77, p < .0001), peak stroke volume (r = .67, p < .005), and peak ejection fraction (r = .69, p < .005). On the other hand, peak filling rate at rest showed a significant inverse correlation with peak end-diastolic (r = -.48, p < .05) and end-systolic (r = -.66, p < .005) volumes. The patients were then subgrouped into two groups according to their rest ejection fraction (lower or higher than 40%). In the group with ejection fraction less than 40% a significant correlation was observed between rest ejection fraction and both peak stroke volume (r = .66, p < .05) and peak ejection fraction (r = .69, p < .05). In the same group of patients an inverse correlation was found between peak filling rate and both end-diastolic (r = -0.65, p < .05) and end-systolic (r = -.82, p < .005) volumes. CONCLUSIONS The results of the present study suggest that exercise capacity is related to left ventricular function at rest and that rest diastolic function might be a determinant of left ventricular function during exercise in patients with heart failure.