Continuous venovenous hemodialysis (CVVHD) or hemofiltration conducted with pre- (CVVHpre) or post- (CVVHpost) dilution modes are recommended to treat patients with acute renal failure (ARF) and cardiovascular instability. The efficiency of the three techniques was compared in a study including 18 critically ill patients with ARF. Their mean age was 62.1 +/- 16.7 years, and their mean SAPS II score was 59.5 +/- 14.3. They were treated sequentially with the three techniques for periods of 24 hours each (randomized assignment to one technique the first 24 hours followed by the two others). The PRISMA device and M 100 (AN69S) membrane were used in all instances. Blood and replacement (or dialysis) flow rates were kept at 150 and 25 ml/min, respectively. Urea, creatinine, uric acid, inorganic phosphorus, beta2 microglobulin (beta2m), and retinol binding protein (RBP) were measured every 12 hours in plasma and in 12 hours filtrate collection for 3 days. The results are expressed as filtrate/mean plasma (F/P) ratio for the 12 hour period. Removal of small molecules was 16% higher using CVVHD and CVVHpost than CVVHpre. For beta2m and RBP, CVVHpre was, respectively, 43% and 26% more efficient than CVVHD. CVVHpost gave higher but statistically different removal than CVVHpre only for beta2m. CVVHpost was the most efficient technique for removal of small proteins, but this advantage could be easily counterbalanced using higher volume substitution.