Objectives Septic acute kidney injury (AKI) requiring continuous renal replacement therapy (CRRT) carries a mortality risk nearing 50%. Risk factors associated with mortality in AKI patients undergoing CRRT with blood culture positive sepsis remain unclear as sepsis has been defined according to consensus criteria in previous studies. Methods Risk factors associated with intensive care unit (ICU), 90-day and overall mortality were studied in a retrospective cohort of 126 patients with blood culture positive sepsis and coincident severe AKI requiring CRRT. Comprehensive laboratory and clinical data were gathered at ICU admission and CRRT initiation. Results 38 different causative pathogens for sepsis and associated AKI were identified. ICU mortality was 30%, 90-day mortality 45% and one-year mortality 50%. Immunosuppression, history of heart failure, APACHE II and SAPS II scores, C-reactive protein and lactate at CRRT initiation were independently associated with mortality in multivariable Cox proportional hazards models. Blood lactate showed good predictive power for ICU mortality in receiver operating characteristic curve analyses with AUCs of 0.76 (95%CI 0.66-0.85) for lactate at ICU admission and 0.84 (95%CI 0.72-0.95) at CRRT initiation. Conclusions Our study shows for the first time that lactate measured at CRRT initiation is predictive of ICU mortality and independently associated with overall mortality in patients with blood culture positive sepsis and AKI requiring CRRT. Microbial etiology for septic AKI requiring CRRT is diverse.