Objective To compare the effect of different diagnostic criteria for acute kidney injury (AKI) from Kidney Disease Improving Global Outcomes (KDIGO) on short-term prognosis in the very elderly inpatients. Methods Clinical data of 652 AKI patients (≥75-year-old, median age 87, ranging from 84 to 91) admitted in our department between January 2007 and December 2015 were collected and retrospectively analyzed in this study. According to the diagnostic window, they were divided into 48-hour diagnostic window group (n=334) and 7-day diagnostic window group (n=318). These patients were also divided into survival (n=433) and death groups (n=219) by their outcomes within 90 d after AKI. Their general conditions and clinical characteristics were compared between the 2 groups. SPSS statistics 19.0 was used to perform the statistical analysis. Student’[KG-*3]s t test, Mann-whitney U test or Chi-square test was used for comparison of different data types between groups. Prognostic survival factors were identified using Cox regression analysis. Kaplan-Meier survival analysis was employed for accumulative survival rate. Results For these 652 enrolled patients, 308 (47.2%) were stratified into stage 1 AKI, 164(25.2%) into stage 2 AKI, and 180(27.6%) into stage 3 AKI according to KDIGO clinical practice guideline. While, 334 cases (51.2%) were diagnosed as AKI by 48-hour diagnostic window, and 318 cases (48.8%) by 7-day diagnostic window. The 90-day mortality was 42.5% in the patients of 48-hour diagnostic window group and 24.2% in those of 7-day diagnostic window group. Kaplan-Meier survival curves showed the 90-day mortality was better in the 7-day diagnostic window group than in the 48-hour diagnostic window group (P<0.001). With the increase of time for AKI occurrence, the 90-day mortality was significantly decreased (P<0.001). Multivariate analysis by the Cox model revealed that low body mass index (HR=0.928, 95%CI:0.886-0.973; P=0.002), low mean arterial pressure (HR=0.969,5%CI:0.959-0.979; P<0.001), low serum prealbumin level (HR=0.948,5%CI:0.920-0.977; P<0.001), low albumin level (HR=0.962, 95%CI:0.930-0.995; P=0.025), infection (HR=1.374,5%CI:1.027-1.840; P=0.033), oliguria (HR=2.069, 95%CI:1.341-3.192; P=0.001), high blood urea nitrogen level (HR=1.027,5%CI:1.015-1.038; P<0.001) and magnesium level (HR=2.485,5%CI:1.351-4.570; P=0.003), more severe AKI stages (stage 2:HR=4.035, 95%CI:2.381-6.837, P<0.001; stage 3:HR=7.184; 95%CI:4.301-11.997, P<0.001), and ≤48-hour window for AKI diagnosis (HR=1.818,5%CI:1.256-2.631; P=0.002) were independent risk factors for 90-day mortality in hospitalized elderly AKI patients. Conclusion The 90-day mortality is higher in 48-hour window AKI than in 7-day window AKI in the very old patients. ≤48-hour window for AKI diagnosis is an independent factor for 90-day mortality for the elderly.