To compare and contrast different heparin regimens for extracorporeal circuit anticoagulation in patients receiving acute continuous hemodiafiltration (ACHD). Prospective controlled randomized comparisons of the following regimens: 1) Low dose (500 IU/h) prefilter heparin versus regional anticoagulation in patients on continuous arteriovenous hemodiafiltration (CAVHD) via A-V shunt. 2) Low dose pre-filter heparin versus no anticoagulation in patients receiving CAVHD via femoral cannulae. 3) Low dose pre-filter heparin versus regional anticoagulation in patients on continuous veno-venous hemodiafiltration (CVVHD). 4) An assessment of the consequences of the use of no anticoagulant in patients predicted to be at high risk of hemorrhagic complications on treatment with CVVHD. University Teaching Hospital ICU. 64 ICU patients with acute renal failure. Haemofilter survival during shunt CAVHD was significantly prolonged by the use of regional anticoagulation compared to the use of low dose heparin (mean filter survival: 57.1 h versus 42.9 h;p<0.05). In CAVHD using femoral cannulae, no significant differences in haemofilter survival were found between anticoagulation with low dose heparin and the use of no anticoagulant (mean filter survival: 55 h versus 52.5 h; NS). During CVVHD, regional anticoagulation compared to low dose heparin produced a trend towards prolonged filter life which was, however, not statistically significant (mean filter survival: 40.5 h versus 31.4 h; NS). In patients assessed to be at high risk of bleeding, CVVHD without anticoagulation provided a mean filter survival of 40.9 h (95% CI 27-54.8 h). Regional anticoagulation leads to longer filter survival than low dose heparin in shunt CAVHD. A regimen of no anticoagulation during femoral CAVHD leads to a filter life similar to that of low dose heparinization. During CVVHD, regional anticoagulation and low dose heparin are associated with similar filter survival times. In patients assessed to be at high risk of bleeding, treatment with CVVHD without anticoagulation results in adequate filter survival.