We studied the haemodynamic effects of inhaled nitric oxide 40 p.p.m. in two groups of patients after cardiac surgery (mitral valve surgery or coronary artery bypass grafting). Nitric oxide caused a significant reduction in pulmonary vascular resistance after mitral valve surgery in patients who had pre-existing pulmonary hypertension, but no change in haemodynamic state in the coronary artery bypass group of patients, who had normal pulmonary arterial pressures.