A retrospective case-series study evaluated the prognosis of 853 dogs with acute kidney injury (AKI) based on the RIFLE (Risk, Injury, Failure, Loss and End-stage renal failure) criteria, derived from human medicine. The 30-day mortality of dogs with AKI in each class was found to be 23.8 per cent (40 of 168) dogs for Risk, 41.0 per cent (107 of 261) dogs for Injury and 78.5 per cent (333 of 424) dogs for Failure. Using the dogs in the Risk class as the reference, the mortality of dogs in either the Injury or Failure class was significantly higher than that of dogs in the Risk class (P<0.05). The longest median survival time was observed in the Risk class (nine days) and the shortest median survival time was observed in the Failure class (three days). Using a multiple logistic regression model, a new score that simultaneously considered RIFLE class, diarrhoea status and serum phosphorus level was calculated to predict prognosis. Evaluation using the area under the receiver-operating characteristic curve (AUROC) indicated that the new scoring method (AUROC 0.80) was a better prognostic indicator than using RIFLE criteria alone (AUROC 0.73).