['The prognostic ability of a single measurement of peak oxygen uptake (VO(2)) is well established in patients with chronic heart failure. The relation between a change in peak VO(2) and clinical outcomes is not well defined.', 'This investigation determined whether an increase in peak VO(2) was associated with a lower risk of the primary end point of time to all-cause mortality or all-cause hospitalization and 3 secondary end points. In Heart Failure and a Controlled Trial to Investigate Outcomes of Exercise Training, an exercise training trial for patients with systolic heart failure, cardiopulmonary exercise tests were performed at baseline and ≈3 months later in 1620 participants. Median peak VO(2) in the combined sample increased from 15.0 (11.9-18.0 Q1-Q3) to 15.4 (12.3-18.7 Q1-Q3) mL·kg(-1)·min(-1). Every 6% increase in peak VO(2,) adjusted for other significant predictors, was associated with a 5% lower risk of the primary end point (hazard ratio=0.95; CI=0.93-0.98; P<0.001); a 4% lower risk of the secondary end point of time to cardiovascular mortality or cardiovascular hospitalization (hazard ratio=0.96; CI=0.94-0.99; P<0.001); an 8% lower risk of cardiovascular mortality or heart failure hospitalization (hazard ratio=0.92; CI=0.88-0.96; P<0.001); and a 7% lower all-cause mortality (hazard ratio=0.93; CI=0.90-0.97; P<0.001).'] Is modest increase in peak VO2 related to better clinical outcomes in chronic heart failure patients : results from heart failure and a controlled trial to investigate outcomes of exercise training?