Oxidative stress occurs when reactive oxygen species overwhelm the body’s antioxidant capacity. Tissue damage and organ dysfunction follow. Cats are particularly susceptible to oxidative stress, with oxidative stress demonstrated in diabetes mellitus, chronic renal failure, and feline immunodeficiency virus. This study investigated whether oxidative stress is present in cats with feline infectious peritonitis (FIP), which causes a severe inflammatory response and a rise in acute phase proteins. Serum samples from cats with a diagnosis of naturally occurring FIP, healthy cats, and cats with other inflammatory conditions were compared in this retrospective study. Serum paraoxonase 1 (PON1) and serum total antioxidant capacity (TAC) were measured. PON1 and TAC are negative biomarkers that decrease during oxidative stress. Signficantly lower PON1 was seen in cats with FIP and other inflammatory diseases. Cats with the effusive form of FIP had lower PON1 and TAC levels than those with the dry form, indicating a greater degree of oxidative stress. Cats with FIP had significantly lower TAC levels than those in the other 2 groups. The authors conclude that oxidative stress is present in cats with FIP; this may have implications for prognosticating, monitoring, and treating FIP.