Hemodialysis, which leads to negative fluid balance, is often required in mechanically ventilated patients with renal failure. We conducted a prospective study on the influence of hemodialysis on the respiratory mechanics in 14 mechanically ventilated patients (10 men, 4 women, mean age 70.1 +/- 8.1 yr) with various causes of renal failure requiring hemodialysis in an intensive care unit. Respiratory mechanics were measured before and after hemodialysis using the interrupter technique under constant flow and constant volume. The values of several parameters of respiratory mechanics were significantly lower after hemodialysis: peak airway pressure (26.97 +/- 4.62 vs 23.45 +/- 4.13 cmH2O), airway pressure at zero flow (23.81 +/- 4.18 vs 21.50 +/- 3.79 cmH2O), airway plateau pressure (18.56 +/- 3.70 vs 16.48 +/- 3.07 cmH2O), and intrinsic positive end-expiratory pressure (4.03 +/- 2.90 vs 2.65 +/- 1.84 cmH2O). Minimal respiratory resistance and maximal respiratory resistance were significantly decreased after hemodialysis (4.38 +/- 1.26 vs 2.99 +/- 1.00 cmH2O/L/s and 9.93 +/- 2.50 vs 8.39 +/- 2.43 cmH2O/L/s, respectively), but effective additional respiratory resistance and static respiratory compliance were not. Significant correlations were found between body weight reduction by hemodialysis and changes in minimal respiratory resistance (r = 0.758, p < 0.01), maximal respiratory resistance (r = 0.544, p < 0.05), and static respiratory compliance (r = 0.703, p < 0.01). We conclude that hemodialysis significantly alters the respiratory mechanics in mechanically ventilated renal failure patients and that the alteration may be related to the negative fluid balance caused by hemodialysis.