CAT and keratoconus: All evaluated biomarkers showed statistically significant differences between subjects with KCN and those without KCN (<i>p</i>-values < 0.001), with higher serum TOS, NOx, MDA, CAT, and oxidative stress index levels and lower levels of TACs and thiols in subjects with KCN than in those without KCN.<h4>Conclusions</h4>Our findings indicate a systemic imbalance between pro-oxidant and anti-oxidant biomarkers in subjects with keratoconus.