The increased production of reactive oxygen species (ROS) from activated neutrophils in Behçet's disease (BD) and recurrent aphthous stomatitis (RAS) may result in increased oxidative stress. Uric acid can react rapidly with neutrophil-derived ROS to form allantoin. The purpose of the study was to evaluate the serum levels of allantoin as a new marker of oxidative stress in BD compared with malondialdehyde (MDA) levels as a well-known marker. Blood samples were obtained from 23 BD patients, 22 RAS patients as positive controls, and 21 healthy controls. When compared to the healthy controls, we found higher allantoin and MDA levels in the BD patients and higher MDA levels in the RAS patients. Serum ascorbic acid levels in the BD patients were significantly lower than in the controls. Increased allantoin and MDA levels suggest the possible involvement of free radicals in BD. As allantoin is only a product of uric acid oxidation by reactive oxygen and nitrogen species, it may also be used as a marker of oxidative stress in BD.