Maximal exercise performance in patients with chronic heart failure, as determined by peak oxygen consumption (VO2max.) during exercise testing has been shown to correlate well with mortality and its use as primary determining factor in the selection of patients for heart transplant has been advocated. Patients with a VO2max of less than 14 ml/kg/min appear at particularly high risk for mortality. Exercise performance can be influenced by a number of independent factors including subject motivation, peripheral deconditioning or other intrinsic abnormalities of skeletal muscle, and primary respiratory disease. It appears to be a precise gauge of functional impairment. VO2max can be used to accurately and reproducibly place an individual in a predefined functional class.