The New York Heart Association (NYHA) Classes are used to appraise the status of patients with heart disease and evaluate treatment outcomes in clinical and research settings. Ambiguity exists concerning the construct the Classes represent and the optimal way to capture and interpret the information. This article examines the NYHA Classes within the context of a published functional status framework by Leidy. The framework proposes that (1) physiological indicators provide information on capacity, (2) physical activity characterizes performance, and (3) symptoms accompanying activity offer insight into reserve. It is proposed that the NYHA Classes provide a summary statement of both the reduction in reserve accompanying a decline in capacity and the concomitant increase in capacity utilization required to maintain performance in patients with heart disease. This premise is illustrated quantitatively through secondary analysis of data from 22 patients with ischemic heart disease and left ventricular dysfunction.