It has been postulated that patients with chronic renal failure, even in the absence of cardiopulmonary symptoms, accumulate interstitial pulmonary fluid, which is removed by haemodialysis. To test this hypothesis we used the indocyanine green (ICG)-heavy water double indicator dilution method to measure lung water, cardiac output, and central blood volume in relation to haemodialysis. Ten uraemic patients, without cardiopulmonary symptoms, were investigated at the beginning and end, and 2 h after, a regular dialysis session. A group of 18 surgical patients about to undergo elective abdominal surgery served as controls. Despite normal gas exchange, central blood volume, and cardiac output at the start of dialysis the mean (SD) lung water was significantly higher than in the control group [4.8 (0.9) compared with 3.6 (0.7) ml/kg, P < 0.001]. There was no correlation between weight gain between sessions of dialysis and the magnitude of lung water at the start of dialysis. Lung water decreased (P < 0.001) to the level of the control group in response to dialysis. There was no correlation between weight loss and reduction in lung water induced by dialysis. In conclusion, we have verified the presence of subclinical pulmonary oedema which was removed by dialysis in a group of patients with established renal failure. The variations in lung water cannot be explained by hydrostatic mechanisms alone.