In moderate congestive heart failure pulmonary overhydration may be detected at chest X-ray even if therapy is optimized to keep the urinary output normal and to prevent weight gain and dependent edema formation. Removal of overhydration of the lung may help to define its significance. This study was aimed at investigating whether a subclinical accumulation of fluid in the lung interstitium in moderate congestive heart failure interferes with the patient's functional capacity, and whether furosemide is able to promote reabsorption of the excessive fluid. Patients whose digoxin, oral furosemide and ACE-inhibitor therapeutic regimen was kept constant, were randomly allocated to ultrafiltration (8 cases) or iv bolus (mean dose = 248 mg) of supplemental furosemide (8 cases). The amount of body fluid removed with each method approximated 1.600 ml. Functional performance was assessed with cardiopulmonary exercise tests. Soon after fluid withdrawal with either procedure the filling pressures of the two ventricles and body weight were reduced and plasma renin activity, norepinephrine and aldosterone were augmented. After furosemide hormones remained elevated in the subsequent 4 days, and, during this period, patients had positive water metabolism, recovery of the elevated ventricular filling pressures, recurrence of lung congestion without any improvement in functional capacity. In ultrafiltrated patients, renin, norepinephrine and aldosterone fell below control values within the first 48 hours and water metabolism was equilibrated at a new set point (less fluid intake and diuresis without weight gain). Functional capacity in these patients was improved through favorable circulatory and ventilatory adjustments consequent on reabsorption of lung water. This may also have restored the ability of the lung to clear norepinephrine, thus restraining its facilitation of renin release. Improvement persisted at 3 months after the procedure. In congestive heart failure the set point of fluid balance is altered despite oral furosemide; supplemental iv furosemide does not shift the set point, at least in the presence of ACE-inhibition; excessive, although silent, lung water limits the functional capacity of the patient.