Background: Acute kidney injury (AKI) places a heavy burden on the healthcare system in China and is usually misdiagnosed. However, there are limited studies that have described the epidemiology and diagnosis of AKI in China. The aim of this study was to describe the incidence and diagnosis of AKI in hospitalized adult patients in a tertiary teaching hospital in southeast China. Methods: All adult patients hospitalized from October 1, 2013 to September 30, 2014 in the First Affiliated Hospital of Nanjing Medical University were screened using the Lab Administration Network. AKI definition and staging were based on the KDIGO AKI criteria. Demographic characteristics, laboratory examination, clinical data, and clinical outcomes of AKI patients were recorded and analyzed. Results: The incidence of AKI was 1.6% (1401/87196). The 30-day mortality was 35.3%. AKI stage 1, 2, 3 and RRT accounted for 38.0% (532/1401), 22.0% (309/1401), 40.0% (560/1401), and 16.3% (228/1401) of patients, respectively. The Renal, other Internal Medicine, Surgery, and ICU Departments accounted for 7.4%, 37.1%, 30.1%, and 25.4% of AKI patients, respectively. The timely diagnosis rate, delayed diagnosis rate, and missed diagnosis rate were 44% (616/1401), 3.3% (46/1401), and 52.7% (739/1401), respectively. Patients hospitalized in the Renal Department had the highest AKI diagnosis rate (89.3%, 88/103), while missed diagnosis rate of the surgical patients was as high as 75.1% (317/422). Multivariable logistic regression analysis indicated that presence of tumors, higher serum albumin, and AKI stage 1 were associated with failure to timely diagnose AKI, whereas presence of chronic kidney disease, oliguria, higher blood urea nitrogen, and greater number of organ failures correlated with earlier diagnosis. Conclusions: AKI was characterized by a high incidence, high short-term mortality, and high missed diagnosis rate in hospitalized adult patients in our hospital. Interventions for improving diagnosis of AKI are urgently needed.