The optimal timing to start continuous renal replacement therapy (CRRT) for acute kidney injury (AKI) in critically ill ICU patients has not been accurately estimated. The proposed risk, injury, failure, loss, end-stage kidney disease (RIFLE) criteria for diagnosis of AKI may provide a method for nephrologists to decide the ‘optimal timing’ for starting dialysis. Objective Our study aimed to analyze the correlation between RIFLE stages at the start of CRRT and 90-day survival rate and to detect the effect of the timing of CRRT on poor kidney outcome in 90-day survivors. Patients and methods A retrospective cohort analysis was performed on the data of 96 critically ill patients with AKI in ICU treated with CRRT during a 2-year period in international extended care hospital, Jeddah, Saudi Arabia, from January 2015 to January 2017. Information such as age, sex, RIFLE stage, sepsis, sepsis-related organ failure assessment score, number of organ failures before CRRT, CRRT time, survival, and kidney outcome conditions at 90 days after CRRT started was collected. According to their baseline severity of AKI at the start of CRRT, the patients were assigned to three groups according to the increasing severity of RIFLE stages, group I (risk of renal dysfunction, R), group II (injury to the kidney, I), and group III (failure of kidney function, F), using RIFLE criteria. The poor kidney outcome was classified as RIFLE-L (loss of kidney function, L) or RIFLE-E (end-stage kidney disease, E) using RIFLE criteria. The correlation between RIFLE stage and 90-day survival rate was analyzed among these three groups. Additionally, the association between RIFLE stage and the poor kidney outcome (RIFLE-L+RIFLF-E) in the 90- day survivors was analyzed.