The instantaneous pattern of pulmonary capillary blood flow was recorded by the nitrous oxide-body-plethysmograph method in 36 patients with heart disease. It was found to be abnormal in the presence of significant pulmonary arterial or pulmonary venous hypertension. Tachycardia, a low cardiac output, and right ventricular failure also contributed to converting a normally pulsatile capillary blood flow pattern to a more uniform one.