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Protein and amine halogenation is a type of oxidative stress induced by phagocytic overstimulation, and its role in Parkinson's disease (PD) has not been discerned. We have detected that advanced oxidized protein products, markers of protein halogenation, are reliably enhanced in serum of patients with PD (n=60) relative to control subjects (n=45, p<0.012), and to a lesser extent in the cerebrospinal fluid. Amine halogenation, as evaluated through 3-chlorotyrosine, is not affected. Mieloperoxidase and hydrogen peroxide levels, halogenative factors of phagocytes, are devoid of changes. Levels of advanced oxidized protein products are progressively reduced over time, and the duration of PD is larger in the Hoehn-Yahr-stage-2/3 patients (n=34) with low serum levels (R(2)=0.0145, p<0.003). Levodopa treatment contributes to this reduction (R(2)=0.259, p<0.001). These protein products are not cytotoxic, unlike 3-chlorotyrosine, but they are known to form inflammatory mediators after conjugation with serum albumin. Our observations lead to the hypothesis that the serum level of advanced oxidized protein products is a prognostic marker of PD duration, and these oxidized proteins could participate in the development of parkinsonian neurodegeneration.