The volume of blood in the vascular bed of the lungs can be estimated, by the Stewart-Hamilton principle, as the product of cardiac output and pulmonary mean transit time. Previous investigators have measured this time indirectly as the difference between transit times from the pulmonary artery and the left atrium to a peripheral artery. The present study was an investigation of the feasibility of measurement of pulmonary blood volume by the simpler and more direct approach of sampling a single dilution curve from the left atrium after injection of indicator into the pulmonary artery. Fifteen measurements were obtained in 10 patients with valvular heart disease. In all cases the atrial dilution curves exhibited satisfactory contours and presented no difficulties in discernment of a single discrete segment of exponential decay. There was an excellent correlation (r = +0.98) between cardiac outputs measured from atrial curves and those measured from simultaneous aortic or arterial dilution curves. Pulmonary blood volumes ranged from 236 to 403 ml./M. 2, with a mean of 311 ml./M. 2, representing 11 per cent of estimated total blood volume. This value differs insignificantly from the results obtained by others who used indirect measurements of pulmonary artery-to-left atrium transit time, but is appreciably smaller than earlier estimates of “central” or “intrathoracic” blood volume obtained from single arterial dilution curves or from isotopic techniques with precordial counting. Reproducibility was excellent: the largest scatter around the mean of a pair of measurements of pulmonary blood volume was ±8 ml./M. 2. There was a significant correlation (r = + 0.73, p < 0.02) between pulmonary blood volume and stroke volume. It is concluded: (1) that dilution curves sampled from the left atrium after injection of indicator into the pulmonary artery provide valid measurements of cardiac output and permit reproducible estimates of pulmonary blood volume, and (2) that the volume of blood in the vascular bed of the lungs is quite constant in the resting steady state, amounts to approximately 11 per cent of total blood volume, and may play a role in the regulation of stroke volume.