It is unknown whether continuous renal replacement techniques result in diminished morbidity and mortality when compared to conventional dialytic techniques. To investigate this issue a previously described, retrospectively studied group of critically ill patients with severe acute renal failure treated by conventional dialysis (CD) was compared to a prospectively studied group of similar patients treated by acute continuous hemodiafiltration (ACHD). A combined retrospective and prospective clinical and laboratory investigation was carried out for 234 consecutive critically ill patients with severe acute renal failure in the intensive care unit of a tertiary institution. Biochemical, clinical and outcome data in all patients treated by conventional dialytic techniques (intermittent hemodialysis and/or peritoneal dialysis) during a 5-year period were retrospectively analyzed, and a prospective analysis of the same biochemical, clinical and outcome data in all patients treated by acute continous hemodiafiltration was done over a similar time span, with statistical comparison of findings. One hundred and fifty patients were treated by ACHD and 84 by CD. ACHD patients were more severely ill (mean APACHE II score: 28.2 vs. 25.8; p < 0.01) and older (mean age: 59.9 vs. 55.5 years; p < 0.01). There were no significant differences in the incidence of sepsis, bacteremia and need for mechanical ventilation. ACHD resulted in better control of uremia (mean steady-state plasma urea level: 20.1 vs. 31.7 mmol/l; p < 0.001) and hyperphosphatemia (mean serum phosphate: 1.26 vs. 1.95 mmol/l) after 24 h of initiation of therapy. It also allowed the administration of full nutritional support in a significantly greater percentage of patients (91.3 vs. 64.8%; p < 0.001). Survival to ICU discharge was significantly greater in ACHD patients (43.3 vs. 29.8%; p < 0.05), but survival to hospital discharge was not statistically different (ACHD: 38.6% vs. CD: 29.8%; n.s.). When survival to hospital discharge was corrected for illness severity, patients with an intermediate degree of illness severity (APACHE score between 19 and 29) were more likely to survive if treated with ACHD rather than CD (50.6 vs. 30%; p < 0.025). ACHD survivors had a shorter mean ICU stay (11.8 vs. 16.9 days; p < 0.05) and a shorter mean duration of hospital stay (33.9 vs. 58.4 days; p < 0.001). The findings of this study suggest that, in critically ill patients, ACHD may provide better control of uremia and a greater ability to administer full nutritional support than CD. They also suggest that the use of ACHD is associated with a shorter duration of ICU and hospital stay and may even provide a survival advantage. The current study invites further investigation of the use of continuous hemofiltration techniques in the critically ill.