To compare and contrast the clinical outcomes in critically ill patients with acute renal failure managed with either acute continuous hemodiafiltration or conventional dialytic therapies.Design: Retrospective review of the medical records of 167 consecutive cases of acute renal failure treated at a single center (July 1982-July 1991). Scoring for illness severity (APACHE II, number of failing organs) and assessment of outcome in terms of biochemical control ofazo-temia, ARF therapy-related morbidity, and overall morbidity and mortality. Setting: Tertiary institution.Patients: 767 consecutive critically ill patients with multiorgan failure and acute renal failure.Measurements and Main Results: 84 patients received conventional dialytic therapy (CDT) (1982–1988) and 83 acute continuous hemodiafiltration (ACHD) (1988–1991). The etiology of ARF and illness severity indices were similar in both groups (organ failure scores: CDT 3.9 vs. ACHD 4.1; NS). All patients were critically ill, with more severely ill patients within the ACHD groups (mean APACHE II score: CDT 25.8 vs. ACHD 28.1; p<. 01). There were no significant differences in pretreatment serum creatinine, glucose, bicarbonate and phosphate, white cell and platelet counts, incidence of disseminated intravascular coagulation, prevalence of sepsis, or evidence of pulmonary and/or peripheral edema. Overall survival was 29.8% for the CDT groups and 41% for the ACHD group (NS). When patients were stratified by severity of illness, survival in those with 2 to 4 failing organs was significantly greater in the ACHD group (CDT 31.1% vs. ACHD 53.8%; p<. 025). Similarly, overall survival in patients with intermediate APACHE II scores (24 to 29) was significantly better in those treated with ACHD (CDT 12.5%vs. ACHD 46.4%; p<. 025). During the course of ARF, in comparison to CDT, ACHD was associated with greater overall reductions in serum creatinine, and in phosphate and plasma urea, and an increased net nutritional intake. Conclusions: ACHD provided biochemical and outcome indicator advantages over conventional dialytic therapy. In patients with 2 to 4 failing organs or an intermediate APACHE II score (24 to 29) a significant survival advantage was demonstrated for ACHD over CDT. Although this study is a retrospective analysis, with all the inherent limitations of such studies, it suggests that ACHD is the treatment of choice for ARF in the critically ill, with maximum benefits seen in those with 2 to 4 failing organs and/or intermediate APACHE II scores.