Feline coronavirus (FCoV) is a highly contagious virus that is ubiquitous in multicat environments and may induce oxidative stress. This virus commonly causes an asymptomatic infection, which can persist in certain individuals. Sporadically and unpredictably, FCoV infection leads to feline infectious peritonitis (FIP), a highly fatal systemic immune-mediated disease. There are no data in the veterinary literature relating to oxidative stress in FCoV. Antioxidant capacity (TAC) can be attributed to single components in the defence systems against free radicals. The measurement of the total oxidant status (TOS) accurately reflects the oxidative status of blood plasma or serum. Nitric oxide (NO) acts as a free radical and contributes to host defences against oxidation. Malondialdehyde (MDA) is a reliable and commonly used marker of overall lipid peroxidation levels and the presence of oxidative stress. This study aimed to determine levels of oxidative stress markers, serum TAC, total oxidant capacity (TOC), NO and serum MDA in 24 cats seropositive for FCoVs and 15 cats seronegative for FCoVs. Significantly higher serum TOC, NO and MDA levels were found in seropositive animals (P < 0.001, P < 0.05 and P < 0.001, respectively) than in seronegative animals. In contrast, serum TAC levels were found to be significantly lower in seropositive cats compared with seronegative cats (P < 0.001). The results of the present study suggest that FCoV seropositivity is associated with oxidative stress and decreased antioxidant status.