Determining whether a patient's symptoms are the result of heart or lung disease requires an understanding of the influence of pulmonary venous hypertension on lung function. Herein, we describe the effects of acute and chronic elevations of pulmonary venous pressure on the mechanical and gas-exchanging properties of the lung. The mechanisms responsible for various symptoms of congestive heart failure are described, and the significance of sleep-disordered breathing in patients with heart disease is considered. While the initial clinical evaluation of patients with dyspnea is imprecise, measurement of B-type natriuretic peptide levels may prove useful in this setting.