In 1954-1955, the author served as a research fellow in the cardiopulmonary laboratory led by André Cournand at Bellevue Hospital in New York. Cournand was a pulmonary physiologist and a professor of medicine at Columbia University. In his quest to obtain mixed venous blood to calculate pulmonary blood flow, he catheterized the right atrium, right ventricle, and pulmonary artery in patients and also measured the pressures in these chambers. Cournand and his collaborators soon appreciated the enormous potential of cardiac catheterization in deepening the understanding of cardiovascular pathophysiology. After a series of groundbreaking studies, Cournand and his coworker Dickinson Richards, as well as German physician Werner Forssmann, were awarded a Nobel Prize in 1956. Cournand's laboratory, his work habits, and his rigorous approach to science are described, as well as the stimulation the author received during the author's fellowship. As a result, the author went on to extend to the left side of the heart the observations that the Cournand group had conducted in the right heart. Also, the author continued Cournand's work on heart failure by developing techniques to measure ventricular function in patients and to describe the neurohumoral changes that occur in human heart failure.