BACKGROUND: Some patients with markedly reduced ejection fractions (EFs) (<35%) have preserved exercise performance greater than predicted for age and gender. Because diastolic function may be a determinant of exercise performance, this study was conducted to test the hypothesis that patients with preserved exercise tolerance despite EFs < 35% may have relatively normal diastolic function. METHODS: Sixty-five subjects with EFs < 35% who underwent exercise Doppler echocardiography and had no inducible ischemia were retrospectively examined. Forty-five subjects with normal EFs (>60%) and preserved exercise capacity were analyzed as a control group. RESULTS: Sixteen of 65 patients with EFs < 35% had greater than predicted normal exercise capacity for their age and gender, and the remaining 49 patients had reduced exercise capacity. Patients with reduced EFs and preserved exercise capacity had E/e' ratios (mean, 10 ± 4) similar to those of control subjects (mean, 10 ± 3) and lower than those with reduced exercise tolerance (mean, 16 ± 8) (P < .01). In addition, they had better diastolic filling patterns and smaller left atrial sizes than patients with EFs < 35% and reduced exercise capacity. Multivariate logistic regression analyses indicated that E/e' ratio was an independent predictor of preserved exercise capacity in patients with reduced EFs. CONCLUSIONS: Relatively intact diastolic function contributes to preserved exercise capacity in patients with reduced EFs (<35%).