In humans, neutrophil:lymphocyte ratio (NLR) and platelet:lymphocyte ratio (PLR) are prognostic markers in cancer and inflammatory processes. This prospective study evaluated whether NLR and PLR were correlated with severity and clinical outcomes in cats (n = 41) and dogs (n = 67) with acute pancreatitis. Diagnosis of pancreatitis was based on clinical signs, elevated canine or feline pancreas-specific lipase, and ultrasound findings. Days from diagnosis to clinical recovery were tabulated, and severity of disease was assessed using a standardized index adapted from human medicine. Study results demonstrated that NLR and PLR were significantly higher in dogs and cats with pancreatitis versus healthy control patients; however, there was no correlation with disease severity. Increased PLR was associated with a longer recovery time in both species. NLR and PLR may provide useful information regarding the course of pancreatitis in cats and dogs.