We sought to assess the prognostic value of peak exercise oxygen consumption (peak VO2) in patients with severe left ventricular systolic dysfunction and mild to moderate symptoms of chronic heart failure. We focused on 1-year mortality. We prospectively studied 77 patients with left ventricular ejection fraction (EF) < or = 25% and NYHA functional class I/II (61%) or III (39%). All patients underwent cardiopulmonary exercise test, two-dimensional echocardiography and 24-hour Holter monitoring. Examined variables were age, etiology, NYHA functional class, EF, peak VO2, and presence of nonsustained ventricular tachycardia. Overall 1-year mortality rate was 23%. At univariate analysis, age > or = 60 years, ischemic etiology, and peak VO2 < 14 ml/kg/min were significantly associated with mortality. At multivariate analysis, peak VO2 was the most powerful predictor of death (p = 0.0001). In the subgroup of patients with a peak VO2 < 14 ml/kg/min, the actuarial 1-year mortality rate was 56%. One additional patient underwent heart transplantation because of severe hemodynamic deterioration. By contrast, in the subgroup of patient with a peak VO2 > 14 ml/kg/min, 1-year mortality rate was 11%. This study provides evidence that patients with severe left ventricular dysfunction and mild to moderate symptoms of chronic heart failure can be accurately stratified into subgroups with strikingly divergent prognosis by an objective criteria such as peak VO2.