The aim of this study was to evaluate the levels of haematological indices in relation to clinical severity in the cats with feline infectious peritonitis (FIP). Fifteen cats with FIP were included in the study, and cats with an albumin-to-globulin ratio between 0.8 and 0.6 were classified as early FIP groups (n = 8), while those with the ratio below 0.6 were classified as late FIP groups (n = 7). The results of the study indicated that elevated neutrophil count (NEU) could be an important marker in the evaluation of both FIP and FIP severity, whereas neutrophil to lymphocyte ratio (NLR) could only be used to identify cats with FIP and did not change with FIP severity. Finally, this study found that platelet count (PLT) was an important marker in detecting cats only with late FIP. In conclusion, this study demonstrated that the cats with FIP can have different haematological responses depending on the inflammation and severity of the inflammation, and that haematological indices can be used to monitor inflammation. In particular, NEU, NLR, and PLT counts were found to be promising indicators in this regard.