The pressure gradient technique was used to evaluate effects of respiration on left ventricular stroke volume in 22 patients: 11 normal patients; eight patients with airway obstruction; and three patients with pericardial tamponade. In normals, stroke volume, systolic pressure, and pulse pressure fell an average of 7, 3 and 11% (P < 0.01), respectively, during inspiration. In patients with airway obstruction, these parameters decreased by 25, 12 and 23% (P < 0.001), respectively. After breath holding, stroke volume also fell immediately with the onset of inspiration in both groups. These results are consistent with a reduction in left ventricular filling during inspiration as the factor primarily responsible for the fall in stroke volume. In patients with pericardial tamponade, variations in left ventricular stroke volume, systolic pressure and pulse pressure were related to: an immediate fall in stroke volume with the onset of inspiration; and a subsequent increase in stroke volume presumably due to an inspiratory increase in right ventricular stroke volume.