Background: Continuous renal replacement therapy (CRRT) is essential in the management of critically ill patients with acute kidney injury (AKI). However the optimal timing for initiating CRRT remains controversial, especially in elderly patients. Therefore we investigated outcomes of early initiation of CRRT in elderly patients with AKI. Method: A total of 616 patients aged equal or over 65 years who started CRRT due to AKI from August 2009 to December 2013 were enrolled prospectively at three centers. They were divided into 2 groups based on the median 6-hours urine output immediately before CRRT was applied. Results: The mean age of both group was 74.3 years. The median 6-hours urine output was 80 mL. 186 patients (60.0%) were male in early initiation group and 179 patients (60.3%) in late initiation group. The most common cause of AKI was sepsis (45.9% versus 46.0%). Mean arterial pressure was higher in early initiation group (79.8 mmHg versus 76.8 mmHg). Prothrombin time-international normalized ratio, total bilirubin, aspartate aminotransferase and alanine aminotransferase were lower in early initiation group (p<0.05). Overall cumulative survival and 28-days survival rates were higher in early initiation group (Log-rank p<0.001 for both). Furthermore, early CRRT treatment was associated with lower mortality rate after adjustment for age, sex, mean arterial pressure, Charlson comorbidity index, Sequential Organ Failure Assessment score, hemoglobin, serum C-reactive protein, serum albumin level and prothrombin time (hazard ratio, 0.752; 95% confidence interval 0.595-0.951, p=0.008). Conclusion: Early initiation of CRRT was associated with a better prognosis in critically ill elderly patients with AKI.