Patients with asthma, allergic rhinitis, or atopic dermatitis experience increased oxidative stress. We conducted a prospective study to examine the levels of advanced oxidation protein products (AOPPs) as an indicator of oxidative stress in 97 patients with allergic rhinitis who were followed in our clinic during a 3.5-month period. Of these 97 patients, 51 were treated with subcutaneous immunotherapy (SCIT), and 46 did not receive any treatment until the study was concluded. In each patient, allergic rhinitis and allergic sensitization were documented by the history, the findings on clinical examination, and the results of blood and skin-prick tests. Blood samples from each patient were analyzed to determine AOPP levels. We found that the mean serum AOPP level was significantly higher in the SCIT group than in the non-SCIT group (258.55 vs. 163.83 µmol/L; p = 0.0015). We conclude that as a known indicator of protein oxidation, the serum AOPP level is a marker of increased oxidative stress in response to allergen exposure in allergic rhinitis.