To investigate the epidemiological characteristics and prognosis of critically ill patients with sepsis combined with acute kidney injury (AKI) in intensive care unit (ICU) in Beijing, and to analyze the risk factors associated with in-hospital mortality among these critically ill patients.Data were collected from the Beijing AKI Trial (BAKIT) database, including 9 049 patients consecutively admitted to 30 ICUs in 28 tertiary hospitals in Beijing from March 1 to August 31, 2012. Patients were divided into non-AKI and non-sepsis group, AKI and non-sepsis group, non-AKI and sepsis group, AKI and sepsis group. Clinical data recorded included demographic characteristics, primary reasons for ICU admission, comorbidities, sequential organ failure assessment (SOFA), acute physiology and chronic health evaluation II(APACHE II) within 24 hours of ICU admission, physiological and laboratory indexes, treatment in the ICU, AKI staging based on the Kidney Disease: Improving Global Outcomes (KDIGO), as well as the prognostic indicators including length of stay in ICU, length of stay in hospital, ICU and in-hospital mortality. The primary endpoint was discharge or in-hospital death. Multivariate Logistic regression analysis was used to investigate the risk factors for hospital death in ICU patients. Kaplan-Meier survival curve was drawn to analyze the cumulative survival of ICU patients during hospitalization.A total of 3 107 critically ill patients were ultimately enrolled, including 1 259 cases in the non-AKI and non-sepsis group, 931 cases in the AKI and non-sepsis group, 264 cases in the non-AKI and sepsis groups, and 653 cases in the AKI and sepsis group. Compared with the other three group, patients in the AKI and sepsis group were the oldest, had the lowest mean arterial pressure (MAP), and the highest APACHE II score, SOFA score, blood urea nitrogen (BUN), and serum creatinine (SCr) levels, and they also had the highest proportion of receiving mechanical ventilation, requiring vasopressor support, and undergoing renal replacement therapy (RRT), all P < 0.01. Of these 3 107 patients, 1 584 (51.0%) were diagnosed with AKI, and the incidence of AKI in patients with sepsis was significantly higher than in those without sepsis [71.2% (653/917) vs. 42.5% (931/2 190), P < 0.01]. The highest proportion of KDIGO 0 stage was observed in the non-sepsis group (57.5%), while the highest proportion of KDIGO 3 stage was observed in the sepsis group (32.2%). Within the same KDIGO stage, the mortality of patients with sepsis was significantly higher than that of non-sepsis patients (0 stage: 17.8% vs. 3.1%, 1 stage: 36.3% vs. 7.4%, 2 stage: 42.7% vs. 17.1%, 3 stage: 54.6% vs. 28.6%, AKI: 46.1% vs. 14.2%). The ICU mortality (38.7%) and in-hospital mortality (46.1%) in the AKI and sepsis group were significantly higher than those in the other three groups. Kaplan-Meier survival curves further showed that the cumulative survival rate of patients with AKI and sepsis during hospitalization was significantly lower than that of the other three groups (53.9% vs. 96.9%, 85.8%, 82.2%, Log-Rank: χ 2 = 379.901, P < 0.001). Subgroup analysis showed that among surviving patients, length of ICU stay and total length of hospital stay were significantly longer in the AKI and sepsis group than those in the other three groups (both P < 0.01). Multivariate regression analysis showed that age, APACHE II score and SOFA score within 24 hours of ICU admission, coronary heart disease, AKI, sepsis, and AKI combined with sepsis were independent risk factors for ICU mortality in patients (all P < 0.05). After adjusting for covariates, AKI, sepsis, and sepsis combined with AKI were significantly associated with higher ICU and in-hospital mortality, with the highest ICU mortality [adjusted odds ratio (OR) = 14.82, 95% confidence interval (95%CI) was 8.10-27.12; Hosmer-Lemeshow test: P = 0.816] and in-hospital mortality (adjusted OR = 7.40, 95%CI was 4.94-11.08; Hosmer-Lemeshow test: P = 0.708) observed in patients with sepsis combined with AKI.The incidence of AKI is high in sepsis patients, and those with both AKI and sepsis have a higher disease burden, more abnormalities in physiological and laboratory indicators, and significantly increased ICU and in-hospital mortality. Among surviving patients, the length of ICU stay and total length of hospital stay are also longer in the AKI and sepsis group. Age, APACHE II score and SOFA score within 24 hours of ICU admission, coronary heart disease, AKI, and sepsis are independent risk factors for in-hospital mortality in ICU patients.