Uric acid, an antioxidant, is reduced in multiple sclerosis (MS). Patients with gout have a reduced incidence of MS. Optic neuritis (ON), often the first manifestation of MS, is not known to be associated with reduced uric acid. Patients with recent onset of ON were investigated to determine whether uric acid levels were reduced at presentation. Twenty-one patients with ON were included, 17 females and 4 males. The mean (SD) serum uric acid in the ON female group was 184.4 ( +/-55.1) micromol/L (range, 116-309 micromol/L), whilst in the control group it was 235.2 (+/- 50.2) micromol/L (range, 172-381 micromol/L). The difference was statistically significant (chi2 = 8.93, P = 0.003). In the small male cohort, mean (SD) serum uric acid was 305 (+/- 52.1) micromol/L, whilst in the control group it was 328 (+/- 80.4) micromol/L. These differences were not statistically significant. Reduced antioxidant reserve is possibly an early pathogenic mechanism in inflammatory demyelination, and raises the possibility that low uric acid levels could be an indicator of disease activity. Since optic neuropathies of other causes were not investigated, future research needs to determine whether low uric acid represents a unique feature of optic neuritis or is seen in other optic neuropathies.