The New York Heart Association (NYHA) functional classification system was developed to help physicians in clinical practice evaluate the effect of cardiac symptoms on a patient's daily activities. Over time, the role of the NYHA classification system has expanded, and it is now frequently used in clinical research. This review of the literature was undertaken to explore whether the NYHA classes have sufficient validity and reliability to serve as a functional outcome measure in research studies. After exploring its strengths and limitations, we conclude that the NYHA classes are a valid measure of functional status, a concept that is distinct from functional capacity and functional performance. The reproducibility of the NYHA functional classification system has not been established in the literature. Researchers are urged to report the methods for determining NYHA class, the training of raters, and the intra-rater or inter-rater reliability in studies that have multiple raters or measurements. Until the reliability of the NYHA functional classification system is determined, it is prudent to refrain from using the NYHA classes as the sole outcome measure of change in function in research studies of cardiac patients.