The Edwards® lung water computer system uses the thermaldye indicator technique to estimate the lung extravascular fiuid volume (EVLW). The authors tested the effect of changes in cardiac output (CO) on EVLW estimates made with the lung water computer in six dogs anesthetized with halothane. Baseline CO was 2.5 ± 1.3 1/min (mean ± SD); CO subsequently was increased either by 220% or decreased by 70% by either giving 0.5 mg/kg of isoproterenol or increasing the inspired halothane (1–4%), respectively. There was a significant correlation between the estimated EVLW and CO in each animal (P< 0.05) such that a 50% decrease in CO from baseline caused an approximately 40% increase in estimated EVLW. Postmortem examination showed that the lungs were not edematous, even though the lung water computer data indicated that severe pulmonary edema had developed at reduced COs. At increased COs, estimated EVLW decreased. The authors conclude that the Edwards® lung water computer overestimtes lung water, possibly because the thermal indicator diffuses into nonpulmonary as well as pulmonary tissue. The overestimate is greatest at low cardiac outputs.