Peak expiratory flow rates were measured during routine haemodialysis in 18 patients with chronic renal failure who were in receipt of thrice weekly haemodialysis treatment, using both a new cuprophan dialyzer and then the same dialyzer after reprocessing. Acetate buffered dialysate was used on both occasions. The peak expiratory flow rates fell by 10 +/- 0.3%, mean +/- sem, during the first hour of treatment with the new dialyzer, and in seven patients (39%) the fall was greater than 15%. Whereas the reduction in peak expiratory flow rates was significantly less with reuse, 4 +/- 0.1% (P less than 0.5). Similarly, the fall in arterial oxygen tension was also reduced on reuse from 28 +/- 1.2% to 14 +/- 1.3% at 30 min (P less than 0.05) and from 30 +/- 1.4% to 18 +/- 3.1% (P less than 0.05) at 60 min of dialysis. There was also a reduction in the fall in the peripheral platelet count at 30 min of dialysis from 14 +/- 0.8% to 9 +/- 0.4% with reuse (P less than 0.05). However, there was no change in dialysis associated leukopenia with reuse of the dialyzer membrane. These results suggest that reprocessing the dialyzer membrane alters its biocompatibility characteristics resulting in an improvement in biocompatibility and further supports the role of inflammatory cell mediator release in the pathogenesis of dialysis associated hypoxaemia and pulmonary dysfunction during the first hour of dialysis treatment.