OBJECTIVE: To determine whether regional anticoagulation of continuous renal replacement therapy circuits using citrate and calcium prolongs circuit life and/or affects circulating cytokine levels compared with regional anticoagulation using heparin and protamine. DESIGN: Multicenter, parallel group randomized controlled trial. SETTING: Seven ICUs in Australia and New Zealand. PATIENTS: Critically ill adults requiring continuous renal replacement therapy. INTERVENTIONS: Patients were randomized to receive one of two methods of regional circuit anticoagulation: citrate and calcium or heparin and protamine. MEASUREMENTS AND MAIN RESULTS: The primary outcome was functional circuit life measured in hours, assessed using repeated events survival analysis. In addition, we measured changes in interleukin-6, interleukin-8, and interleukin-10 blood levels. We randomized 212 subjects who were treated with 857 continuous renal replacement therapy circuits (median 2 circuits per patient [interquartile range, 1-6], 390 in citrate group vs 467 in heparin group). The groups were well matched for baseline characteristics. Patients receiving regional continuous renal replacement therapy anticoagulation with heparin and protamine were more likely to experience circuit clotting than those receiving citrate and calcium (hazard ratio, 2.03 [1.36-3.03]; p <0.0005; 857 circuits). The median lifespan of the first study circuit in each patient was 39.2 hours (95 CI, 32.1-48.0?hr) in the citrate and calcium group versus 22.8 hours (95 CI, 13.3-34.0?hr) in the heparin and protamine group (log rank p = 0.0037, 204 circuits). Circuit anticoagulation with citrate and calcium had similar effects on cytokine levels compared with heparin and protamine anticoagulation. There were more adverse events in the group assigned to heparin and protamine anticoagulation (11 vs 2; p = 0.011). CONCLUSIONS: Regional citrate and calcium anticoagulation prolongs continuous renal replacement therapy circuit life compared with regional heparin and protamine anticoagulation, does not affect cytokine levels, and is associated with fewer adverse events.