Free water handling is impaired in chronic obstructive lung disease (COLD). We studied the role of AVP in this mechanism with a sensitive urinary AVP radioimmunoassay (RIA) in 13 patients with CF. The mean age was 14.7 +/- 1.27 SE years. The control group was age and sex matched. While the 24-hour urine volume was not significant different in both groups, urine osmolality was higher in CF (788 mosmol/kg +/- 182 SE vs. 622 mosmol/kg +/- 218 SE; p = 0.05). The 24-hour urinary AVP excretion in CF patients was found to be significantly elevated (174 ng/day +/- 99 SE vs. 74 ng/day +/- 48 SE; p = 0.01). This difference becomes even greater if these values are corrected for body weight (4.4 ng/(kg X day) +/- 2.1 SE vs. 1.5 ng/(kg X day) +/- 0.9 SE; p = 0.005). Lung function studies were available in 8 patients. We used a body plethysmograph to measure FEV1 (1 sec forced expiratory volume) and MEF25 (forced expiratory flow at 25% of vital capacity) as sensitive indices of airway obstruction. There was a significant negative correlation between urinary AVP excretion and MEF25 (r = .76) and FEV1 (r = .65): the more pronounced the airway obstruction the higher the AVP excretion. We conclude that AVP plays a major role in the impaired water handling in COLD. Increased AVP secretion is already present under clinically stable conditions and is dependent on the degree of airway obstruction. In treating those patients one has to be aware of the possibility of water intoxication.