The levels of uric acid (UA), a natural peroxynitrite scavenger, were measured in sera from 240 patients with multiple sclerosis (MS) and 104 sex- and age-matched control patients with other neurological diseases (OND). The mean serum UA concentration was lower in the MS than in the OND group, but the difference did not reach the level of statistical significance (P = 0.068). However, the mean serum UA level from patients with active MS (202.6 + 67.1 mumol/l) was significantly lower than that in inactive MS patients (226.5 + 78.6 mumol/l; P = 0.046) and OND controls (P = 0.007). We found a significant inverse correlation of serum UA concentration with female gender (P = 0.0001), disease activity (P = 0.012) and duration (P = 0.017), and a trend towards an inverse correlation with disability as assessed by EDSS score, which did not reach statistical significance (P = 0.067). Finally, multivariate linear regression analyses showed that UA concentration was independently correlated with gender (P = 0.0001), disease activity (P = 0.014) and duration of the disease (P = 0.043) in MS patients. These findings suggest that serum UA might serve as a possible marker of disease activity in MS. They also provide support to the potential beneficial therapeutic effect of radical-scavenging substances in MS.