Aim. To estimate the frequency and severity of acute kidney injury (AKI) in patients with ST-segment elevation acute myocardial infarction (STSEAMI), to specify whether the changes in diuresis and serum creatinine levels are equally sensitive diagnostic criteria for AKI, and to define their prognostic value. Subjects and methods. Three hundred and nineteen patients (249 (78%) men and 70 (22%) women; age 58 10 years) with STSEAMI who received thrombolytic therapy (TLT) were examined. The diagnosis of STSEAMI, indications for and contraindications to TLT, evaluation of its efficiency were made in accordance with the All-Russian Scientific Society of Cardiology guidelines (2007). AKI was diagnosed and classified using the KDIGO guidelines (2012). Results. AKI was diagnosed in terms of diuresis, calculated creatinine levels, and creatinine level changes in 107 (34%), 73 (23%), and 68 (22%) patients, respectively. Among the patients with AKI diagnosed in view of diuresis, in-hospital death rates were higher than in those without AKI (X-2=25.46; p<0.001); the similar pattern was seen in patients with AKI diagnosed in terms of calculated creatinine levels (X-2=3.99; p=0.045). Logistic regression analysis indicated that regardless of gender, age, and time interval between onset of clinical manifestation and hospital admission, the in-hospital death rates were associated with the presence of AKI in view of diuresis (relative risk 14; 95% confidence interval, 4.03 to 52.08; p<0.001). Conclusion. The STSEAMI patents receiving TLT exhibited a high rate of AKI. The major problem in the early detection of AKI is associated with difficulties in the differential diagnosis of AKI and chronic kidney disease. AKI diagnosed in view of diuresis is of greater prognostic value for in-hospital mortality than that diagnosed in terms of creatinine levels. The diagnosis of renal dysfunction in view of basal creatinine levels is prognostically important despite the fact that this cannot differentiate AKI from chronic kidney disease in the early stage.