Urea and creatinine clearances achieved using continuous veno-venous haemofiltration were calculated in 16 critically ill patients, during 50 episodes of filtration. The effects of filter life and the volume of ultrafiltrate on these clearances were also evaluated. Clearances were calculated from urea and creatinine concentrations in blood and ultrafiltrate and the volume of ultrafiltrate produced. The overall mean (SD) urea clearance was 26.6 (6.0) ml.min-1 and the overall creatinine clearance was 30.1 (6.3) ml.min-1. The mean (SD) ultrafiltrate production was 29.6 (5.9) ml.min-1. Creatinine clearance was significantly lower in filters that failed within 24 h (filters < 24 h 27.5 (6.3) ml.min-1; filters > 24 h 32.2 (5.5) ml.min-1). The clearance of both solutes increased with increasing ultrafiltrate volume (p < 0.001). We conclude that satisfactory clearance of urea and creatinine can be achieved using continuous veno-venous haemofiltration. Increases in ultrafiltrate production lead to similar increases in urea and creatinine clearance. Prolongation of filter life may improve creatinine clearance.