The possibility of measuring pulmonary blood flow in patients with central arteriovenous shunts utilizing the initial portion of indicator-dilution curves was explored in 16 patients by comparing these values for pulmonary blood flow with determinations by the direct Fick method. A good correlation between Fick and indicator-dilution values was obtained, and no systematic differences were demonstrated. The standard deviation of the differences between Fick and dye values was in the order of 20 to 25 per cent.