Of 22 patients with the classical clinical signs of pulmonary oedema (orthopnoe, cyanosis, sweating and rales heard at a distance) 15 (Group A) were observed clinically, while seven (Group B) underwent haemodynamic studies. Those in Group A were given 0.8-2.4 mg nitroglycerin sublingually one to six times at 5--10 minute intervals. Within five minutes of nitroglycerin administration 7 of the 15 had their first signs of clinical improvement. In 11 patients the rales had disappeared after 15--20 minutes or had regressed. In the remainder the dyspnoea had decreased so that at this point in 14 of the 15 patients various degrees of improvement had occurred. Four patients were completely without clinical signs after 30 minutes. The increased arterial blood pressure and heart rate had fallen markedly. In the seven patients of Group B mean left ventricular filling pressure fell within ten minutes of nitroglycerin administration (1.6 mg) from 33 +/- 10 to 24 +/- 8 mmHg, cardiac output rising sifnificantly from 3.3 +/- 0.8 to 3.7 +/- 0.8 1/min. Such favourable results with nitroglycerin are to be expected only if the pulmonary oedema is of cardiac origin.