Lung water (LW) reflects the water content of the lung interstitium. Because hemodialysis patients have expanded total body water (TBW) they may also have increased LW. Hypertonic saline promotes a flux of water from lung to blood, which is measured by ultrasound flow probes on hemodialysis tubing. The volume of flux is an indirect measure of LW. Our purpose was to determine the feasibility and reproducibility of LW derived with ultrasound velocity dilution, to determine the effect of ultrafiltration on LW in stable hemodialysis patients, and to compare changes in LW with fluid compartment shifts using bioimpedance. Lung water, cardiac output, total body water, and extracellular and intracellular fluid volumes were measured in 24 stable hemodialysis patients at the beginning of hemodialysis and after ultrafiltration. The LW values at the beginning of hemodialysis (298.8 +/- 90.2 ml or 3.67 +/- 1.47 ml/kg) fell during hemodialysis (250.8 +/- 55.8 ml or 3.12 +/- 0.96 ml/kg; p < 0.05), as did TBW and extracellular fluid volumes (p < 0.001). Cardiac output, cardiac index, and central blood volume also decreased significantly with ultrafiltration (p < 0.005, p < 0.005, and p < 0.01, respectively). Results showed that stable hemodialysis patients have higher specific LW values (3.67 ml/kg) than the normal population (2 ml/kg) and ultrafiltration produces a significant decline in LW values.