Objective To study polyuria during mechanical ventilation in patients with chronic obstructive pulmonary disease ( COPD) and its possible mechanisms. Methods The plasma and urine concentrations of cystatin C ( CyC) were measured by enzyme immunometric assay in 26 COPD patients who received mechanical ventilation at timepoints of 0 h, 24 h, 48 h, 72 h and 1 week. And plasma levels of atrial natriuretic factor ( ANF ) , antidiuretic hormone ( ADH) and aldosterone ( ALD) were detected byradioimmunoassay at the same time-point. The urine volume was recorded every day. The samples of 30 healthy volunteers were measured as control. Results Polyuria phenomenon was found in majority of patients in 24 to 72 hours after mechanical ventilation. At 0h, the concentrations of plasma CyC, urine CyC,plasma ANF, ADH and ALD were all increased significantly compared with those of the control, respectively [ ( 4. 87 ±0. 51) mg/L vs ( 1. 29 ±0. 27) mg/L, ( 0. 58 ±0. 13) mg/L vs ( 0. 07 ±0. 02) mg/L, ( 37. 02 ±4. 35) pmol /L vs ( 22. 51 ±1. 18) pmol /L, ( 8. 61 ±1. 43) pmol /L vs ( 0.94 ±0. 34) pmol /L, ( 925. 4 ±142. 7) pmol /L vs ( 297. 5 ±135. 8) pmol /L, all P lt; 0. 01] , then decreased gradually after mechanical ventilation. The levels of plasma CyC and ANF at 24 h, plasma ADH at 72 h, and urine CyC at 1 week were similar to those of the control( all P gt;0. 05) , respectively, except the level of plasma ALD was still higher by 1 week( P lt; 0. 05) . Conclusions Polyuria is not a rare phenomenon for COPD patients receiving mechanical ventilation. Polyuria is related to the readjustment and mal-adaptation of ADH and reninangiotensin-aldosterone-systems during mechanical ventilation.