【Objective】 To investigate the value of urinary N-acety-β-D-glucosaminidase (NAG)/creatinine (Cr) ratio for the early diagnosis of acute kidney injury (AKI) in elderly patients with severe community-acquired pneumonia (CAP). Methods We retrospectively analyzed the changes of urinary NAG/Cr ratio in 19 elderly patients with severe CAP and AKI. We also analyzed the ratio in 19 elderly people without renal disease and pneumonia as controls. Urinary NAG and Cr were measured before AKI and after the presence of AKI for 48 hours and 7 days. NAG was assayed by spectrophotometry, and urinary Cr by biochemistry method on automatic analyzer. Results In the 19 patients after presence of AKI for 48 hours, 15 cases were identified as stage I AKI, 3 cases as stage II, and one case as stage III AKI, based on the criteria from the Acute Kidney Injury Network (AKIN). In elderly patients with severe CAP after presence of AKI for 48 hours, urinary NAG/Cr ratio was significantly higher than that before the presence of AKI (P<0.01). In the 12 elderly patients with severe CAP after presence of AKI for 7 days, urinary NAG/Cr ratio was significantly lower in the 7 patients recovering from AKI than the 5 patients having acute renal failure. Conclusion Urinary NAG/Cr ratio may be useful for the early diagnosis of AKI in elderly patients with severe CAP.