Background Acute kidney injury (AKI) is a disease that negatively affects patient prognosis and requires early diagnosis and treatment. Biomarkers that predict AKI are needed for early diagnosis of this disease. Methods We compared the AKI group and the non‐AKI group in patients who were admitted to our critical care intensive care unit (ICU) and conducted a comparative study focusing on urinary neutrophil gelatinase‐associated lipocalin (U‐NGAL) and serum procalcitonin (PCT). Results Seventy‐one out of 106 ICU inpatients were diagnosed with AKI in accordance with the Kidney Disease: Improving Global Outcomes (KDIGO) criteria. Among the patients who were diagnosed with AKI stages 1 to 3, 94.4% of all patients reached the maximum stage by day 5 after admission. Comparing the non‐AKI group and AKI stage 1 to 3 on days 1 to 3 after admission, U‐NGAL and PCT levels in the stage 3 group were significantly higher than those in the non‐AKI group. Additionally, in receiver operating characteristic curve (ROC) analysis on days 1–3 after admission, U‐NGAL and PCT levels can be used as biomarkers for the diagnosis of AKI, and in particular, AKI stage 3 can be predicted and diagnosed with high accuracy. U‐NGAL and PCT levels were also significantly higher in AKI due to sepsis and acute pancreatitis and due to sepsis, respectively. Conclusions Measuring U‐NGAL and PCT levels as biomarkers for AKI may further improve the accuracy of AKI diagnosis in critical care ICU.